Choirboy to shake with fear, having missed Mass (6)
I believe the answer is:
treble
'choirboy' is the definition.
(I've seen this before)
'shake with fear having missed mass' is the wordplay.
'shake with fear' becomes 'tremble' (I've seen this before).
'having missed' indicates named letters should be taken away.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'tremble' with 'm' taken away is 'TREBLE'.
'to' is the link.
(Other definitions for treble that I've seen before include "Inner ring score (darts)" , "Two-channel sound" , "high frequency" , "Feat of winning three major trophies" , "High voice; occurring three times" .)