Just monkey caught by foxy (6)
I believe the answer is:
simply
'just' is the definition.
(synonyms)
'monkey caught by foxy' is the wordplay.
'monkey' becomes 'imp' (synonyms).
'caught by' means one lot of letters goes inside another.
'foxy' becomes 'sly' (synonyms).
'imp' put into 'sly' is 'SIMPLY'.
(Other definitions for simply that I've seen before include "In a straightforward manner" , "Merely, just" , "Without fuss" .)