Pretend to be singing by mouthing the words (3-5)
I believe the answer is:
lip-synch
'pretend to be singing by mouthing the words' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for lip-synch that I've seen before include "Mouth pre-recorded words, eg on film" , "Speak to a pre-recorded soundtrack" , "Pretend to be singing by mouthing the words" .)