Crossword Genius

Keen on headless horse (4)

Ross

I believe the answer is:

into

'keen on' is the definition.
(I've seen this before)

'headless horse' is the wordplay.
'headless' means to remove the first letter.
'horse' becomes 'pinto' (pinto is a kind of horse).
'pinto' with its initial letter taken off is 'INTO'.

(Other definitions for into that I've seen before include "Fascinated by" , "Two -- six is three" , "Teachers' trade union" , "Not I (anag.)" , "Curious about" .)

I've seen this clue in The Telegraph.
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