Chorister swallows first bit of milk shake (7)
I believe the answer is:
tremble
'shake' is the definition.
(I know that tremble is a kind of shake)
'chorister swallows first bit of milk' is the wordplay.
'chorister' becomes 'treble' (I've seen this before).
'swallows' is an insertion indicator.
'first bit of milk' becomes 'm' (1st letter of 'milk').
'treble' going around 'm' is 'TREMBLE'.
(Other definitions for tremble that I've seen before include "Shiver or jerk quickly" , "Look nervous" , "Quakers do this" , "Shake with fear or excitement" , "Shudder" .)