Setter with a certain capacity (7)
I believe the answer is:
measure
'capacity' is the definition.
(thesaurus)
'setter with a certain' is the wordplay.
'setter' becomes 'me' (the setter of the clue).
'with' says to put letters next to each other.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'me'+'a'+'sure'='MEASURE'
(Other definitions for measure that I've seen before include "Established the size" , "Show something's dimensions" , "Act - dance" , "Establish the size" , "bar of music" .)