Good slimmer, one gathers (7)
I believe the answer is:
gleaner
'one gathers' is the definition.
(I've seen this before)
'good slimmer' is the wordplay.
'good' becomes 'g' (abbreviation).
'slimmer' becomes 'leaner' ('lean' can be similar in meaning to 'slim').
'g'+'leaner'='GLEANER'
(Other definitions for gleaner that I've seen before include "farm worker" , "Leftover grain gatherer" , "Description of Ruth" , "Person gathering" , "One gathering (grain) after a harvest" .)