Asking RAF trainee to cover operation (4,5)
I believe the answer is:
skin graft
'operation' is the definition.
(I've seen this before)
'asking raf trainee to cover' is the wordplay.
'to cover' says the answer is hidden in the clue (I've seen 'covering' mean this).
'SKIN GRAFT' is hidden in the letters of 'asking raf trainee'.
(Other definitions for skin graft that I've seen before include "operation" , "Fresh tissue" , "Surgical transplant" .)