Substitute with good reputation (8)
I believe the answer is:
standing
'reputation' is the definition.
(one's standing is one's reputation or status)
'substitute with good' is the wordplay.
'substitute' becomes 'stand in' (synonyms).
'with' means one lot of letters go next to another.
'good' becomes 'g' (abbreviation).
'standin'+'g'='STANDING'
(Other definitions for standing that I've seen before include "Being a candidate - status" , "Upright" , "type of committee" , "(Of corn) not yet reaped" , "Status, reputation" .)