Manage to finish by thinking originally (4)
I believe the answer is:
tend
'manage' is the definition.
(thesaurus)
'finish by thinking originally' is the wordplay.
'finish' becomes 'end' ('end' can be a synonym of 'finish'**).
'by' is a charade indicator (letters next to each other).
'originally' indicates taking the first letters.
The initial letter of 'thinking' is 't'.
'end' after 't' is 'TEND'.
'to' acts as a link.
(Other definitions for tend that I've seen before include "Look after (a patient)" , "Incline; cultivate" , "Have leaning towards - put dent in (4)" , "Look after, take care of" , "See to" .)