Jam pot's first in hamper (5)
I believe the answer is:
cramp
'hamper' is the definition.
(both can mean to frustrate or hinder)
'jam pot's first' is the wordplay.
'jam' becomes 'cram' (synonyms).
'pot's first' becomes 'p' (1st letter of 'pot').
'cram'+'p'='CRAMP'
'in' acts as a link.
(Other definitions for cramp that I've seen before include "writers may be familiar with" , "Sudden painful tightening of a muscle" , "Painful muscular contraction" , "Muscular spasm, very painful" , "To restrict; a muscle pain" .)