Is short of slack, so we hear (5)
I believe the answer is:
lacks
'is short of' is the definition.
(I've seen this in another clue)
'slack so we hear' is the wordplay.
'slack' becomes 'lax' (similar in meaning).
'so we hear' indicates a 'sounds like' (homophone) clue.
'lax' sounds like 'LACKS'.
(Other definitions for lacks that I've seen before include "Needs" , "Is without, wants" , "Doesn't have" , "Has a want" , "Wants, is deficient in" .)