Crushing on (4)
I believe the answer is:
into
'crushing on' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for into that I've seen before include "Keen on (informal)" , "Taking an active and lively interest (in something)" , "Keen on, colloquially" , "Not I (anag.)" , "Curious about" .)