Agile Romeo taken in by mole (4)
I believe the answer is:
spry
'agile' is the definition.
(synonyms)
'romeo taken in by mole' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'taken in by' means one lot of letters goes inside another (back).
'mole' becomes 'spy' (I've seen this before).
'r' put within 'spy' is 'SPRY'.
(Other definitions for spry that I've seen before include "Agile (for one's age?)" , "(Of an old person) lively" , "Alert" , "Energetic" , "Moving quickly and lightly" .)