Entered left by mistake and threw in the towel (8)
I believe the answer is:
relented
'threw in the towel' is the definition.
(to throw in the towel is to give up)
'entered left by mistake' is the wordplay.
'left by' becomes 'l' (I've seen this in other clues).
'mistake' indicates anagramming the letters.
'entered'+'l'='enteredl'
'enteredl' anagrammed gives 'RELENTED'.
'and' acts as a link.
(Other definitions for relented that I've seen before include "Gave in (to pleas?)" , "Give in (to pleas?)" , "thought better of it" , "Grew less severe" , "Became less intense" .)