Queen breaking finger, which is red (4)
I believe the answer is:
trot
'is red' is the definition.
(I've seen this before)
'queen breaking finger' is the wordplay.
'queen' becomes 'r' (abbreviation for regina, e.g. in ER ).
'breaking' is an insertion indicator.
'finger' becomes 'tot' (both can mean small measure).
'r' placed inside 'tot' is 'TROT'.
'which' is the link.
(Other definitions for trot that I've seen before include "Left wing" , "(Horse's) moderate pace" , "Moderate running pace" , "Militant socialist (informal)" , "An elderly woman" .)