Let loose a gun dog in the centre (4)
I believe the answer is:
undo
'let loose' is the definition.
(I've seen this in another clue)
'a gun dog in the centre' is the wordplay.
'a gun dog' becomes 'gundog' (synonyms).
'in the centre' means to look at the middle letters.
The centre of 'gundog' is 'UNDO'.
(Other definitions for undo that I've seen before include "Unfasten or untie" , "(4)" , "Relax" , "Loosen (knot)" , "Unfasten - destroy" .)