A number of answers are related and their clues contain no definition.

ACROSS
1 Mole needs to take shelter (6)
4 Yes, we gad about towards the border (8)
10 A trade union backing Arabic style of despotism (9)
11 Take some exercise? That’s unknown (5)
12 Parish priest gets involved in country dance (5)
13 Making comeback, triumphed with great success in several European capitals (4,5)
15 A cold needs treatment at first, working on one part of the body (8,6)
18 Boiler burns out, requiring repair, but this will get flat functional again? (6,8)
22 A science graduate’s letters will reveal causes of swellings (9)
24 One goes around Romania on trip, having abandoned university (5)
25 Conclude where a street should be located to make it faster? (5)
26 James perhaps will replace Mark in top committee of communists (9)
27 Friend restrains one burning to fight for cause (8)
28 Stockhausen’s not initially simple for the listener (6)

DOWN
1 Wash the wound, removing one hospital bandage (6)
2 City chief’s in charge but not in the right position (7)
3 On which several callers can talk firm political policy? (5,4)
5 Clip that’s cut short (3)
6 Give some women dowries? (5)
7 Made request for a very quiet song (7)
8 Lad’s eating bananas maybe – first off, they’re rich in protein (3,5)
9 Half of material’s associated with allernative names (not American) for Cape gooseberies (10)
14 Only engineer kept apart by struggling boss is becoming redundant (10)
16 One’s charged executive, first of all, with wrong crime (9)
17 Being in newspaper perhaps is maddening at the start (8)
19 University fellow, backtracking, accepts 3? On the contrary (7)
20 Summary dismissal of only child from elocution training  (7)
21 Heartless guy pinches bottom (6)
23 Bird, eagle, oddly takes time crossing river (5)
26 Postman’s Knock on the way back (3)