Oarsman returning with potassium to modify (6)
I believe the answer is:
rework
'to modify' is the definition.
(thesaurus)
'oarsman returning with potassium' is the wordplay.
'oarsman' becomes 'rower' (synonyms).
'returning' says the letters should be written in reverse.
'with' is a charade indicator (letters next to each other).
'potassium' becomes 'K' (K is the chemical symbol for potassium).
'rower' in reverse letter order is 'rewor'.
'rewor'+'k'='REWORK'
(Other definitions for rework that I've seen before include "Fashion again" , "Alter, revise" , "What editors do" .)