Crossword Genius

The chap on piano was 'with it' (3)

Ross

I believe the answer is:

hep

'with it' is the definition.
(old-fashioned spelling of 'hip', meaning trendy)

'the chap on piano' is the wordplay.
'the chap' becomes 'he' ().
'on' is a charade indicator (letters next to each other).
'piano' becomes 'p' (musical abbreviation).
'he'+'p'='HEP'

'was' is the link.

(Another definition for hep that I've seen is " Knowing (jazz slang); rhymes with dance move".)

I've seen this clue in the Evening Standard.
Want a hint initially instead of a full solution? Install my app