Keen on mackintosh coats (4)
I believe the answer is:
into
'keen on' is the definition.
(if you're 'into' something you're keen on it)
'mackintosh coats' is the wordplay.
'coats' indicates a hidden word (I've seen 'coat' mean this (I've seen this in other clues)).
'INTO' is hidden in the letters of 'mackintosh'.
(Other definitions for into that I've seen before include "Knowledgeable about" , "To a point within - not I" , "Four this twelve is three" , "digging" , "Involved with" .)