Massage a digit, mostly in irregular rhythm (6)
I believe the answer is:
rubato
'in irregular rhythm' is the definition.
'massage a digit mostly' is the wordplay.
'massage' becomes 'rub' (I've seen this before).
'digit' becomes 'toe' (toe is a kind of digit).
'mostly' means to remove the last letter.
'toe' with its final letter removed is 'to'.
'rub'+'a'+'to'='RUBATO'
(Other definitions for rubato that I've seen before include "Departure from strict tempo; bar out (anag.)" , "played with freedom" , "not in regular time" , "Tabour (anag.)" , "regardless of tempo" .)