Having lost weight after golf, one picks up (7)
I believe the answer is:
gleaner
'one picks up' is the definition.
(someone who gleans or picks something up)
'having lost weight after golf' is the wordplay.
'having lost weight' becomes 'leaner' (more lean or more thin).
'after' says to put letters next to each other.
'golf' becomes 'g' (phonetic alphabet: alpha, bravo, charlie etc.).
'leaner' put after 'g' is 'GLEANER'.
(Other definitions for gleaner that I've seen before include "Description of Ruth" , "one gathers" , "Person gathering" , "farm worker" , "Collector of leftover grain after a harvest" .)