Mostly disagree about us lacking concentration (7)
I believe the answer is:
diffuse
'lacking concentration' is the definition.
(diffuse means spread out ie not concentrated)
'mostly disagree about us' is the wordplay.
'mostly' means to remove the last letter.
'disagree' becomes 'differ' (synonyms).
'about' is an insertion indicator.
'differ' with its final letter removed is 'diffe'.
'diffe' enclosing 'us' is 'DIFFUSE'.
(Other definitions for diffuse that I've seen before include "Radiate; unfocused" , "Spread out, not concentrated" , "Widely spread" , "Scatter(ed)" , "Circumlocutory" .)