Quick mole devouring rook (4)
I believe the answer is:
spry
'quick' is the definition.
(synonyms)
'mole devouring rook' is the wordplay.
'mole' becomes 'spy' (I've seen this before).
'devouring' means one lot of letters goes inside another.
'rook' becomes 'r' (abbreviation in chess).
'spy' placed around 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Still able to move lightly and fast" , "Far from doddering" , "Light on one's feet" , "Quick and agile" , "Energetic" .)