Advanced in a rush but thought better of it (8)
I believe the answer is:
relented
'thought better of it' is the definition.
(I've seen this before)
'advanced in a rush' is the wordplay.
'advanced' becomes 'lent' (advance can mean to lend).
'in' is an insertion indicator.
'a rush' becomes 'reed' (I've seen this in other clues).
'lent' going inside 'reed' is 'RELENTED'.
'but' is the link.
(Other definitions for relented that I've seen before include "Gave in and became less severe" , "Grew less severe" , "Became less severe, let up" , "thought better of it" , "Gave in (to pleas?)" .)