It's an idea if the cat turns tail first (5)
I believe the answer is:
motif
'it's an idea' is the definition.
(motif is a kind of idea)
'if the cat turns tail first' is the wordplay.
'the cat' becomes 'tom' (a tom is a male cat).
'turns tail' says the letters should be written in reverse.
'first' is a charade indicator (letters next to each other).
'tom' reversed gives 'mot'.
'if' after 'mot' is 'MOTIF'.
(Other definitions for motif that I've seen before include "sign" , "Artistic theme, pattern" , "Recurring pattern" , "Recurrent symbol" , "Theme in a work of art" .)