Film that’s certainly no light work (1,4,4,5)
I believe the answer is:
a hard days night
'film' is the definition.
(I've seen this before)
'certainly no light work' is the wordplay.
I cannot quite see how this works, but
'certainly' could be 'ay' (variant spelling of 'aye') and 'ay' is found in the answer.
'no' could be 'n' (common abbreviation) and 'n' is found within the answer.
'work' could be 'hard' (hard work is a kind of work) and 'hard' is located in the answer.
This explanation may well be incorrect...
'that's' acts as a link.
(Other definitions for a hard days night that I've seen before include "Picture" , "Beatles song, film and album" , "Single" .)