Showing initiative, but regret spin in letters (12)
I believe the answer is:
enterprising
'showing initiative' is the definition.
(I've seen this before)
'regret spin in letters' is the wordplay.
'letters' is an anagram indicator.
'regret'+'spin'+'in'='regretspinin'
'regretspinin' anagrammed gives 'ENTERPRISING'.
'but' is the link.
(Other definitions for enterprising that I've seen before include "Go ahead" , "daring" , "Adventurous and imaginative" , "Go-getting" , "Full of initiative" .)