Fret on seeing good value (5)
I believe the answer is:
grate
'fret' is the definition.
('grate' can be a synonym of 'fret')
'on seeing good value' is the wordplay.
'on' says to put letters next to each other.
'seeing good' becomes 'g' (abbreviation).
'value' becomes 'rate' ('rate' can be a synonym of 'value').
'g' next to 'rate' is 'GRATE'.
(Other definitions for grate that I've seen before include "Reduce to small shreds" , "Barred framework" , "Firebasket" , "Grind (teeth)" , "Rasp; fire holder" .)