Needs to be casual in speech (5)
I believe the answer is:
lacks
'needs' is the definition.
(lack is a kind of need)
'casual in speech' is the wordplay.
'casual' becomes 'lax' (I've seen this before).
'in speech' shows a homophone (sound like).
'lax' is a homophone of 'LACKS'.
'to be' acts as a link.
(Other definitions for lacks that I've seen before include "Wants, is deficient in" , "Is without, wants" , "Has a want" , "Needs" , "Doesn't have" .)