In the East End, he's bound to find the one that got away (7)
I believe the answer is:
escaper
'the one that got away' is the definition.
'in the east end he's bound' is the wordplay.
I cannot really see how this works, but
'east' could be 'e' (synonyms) and 'e' is found in the answer.
'bound' could be 'caper' (I've seen this before) and 'caper' is found in the answer.
A single letter 's' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'to find' acts as a link.
(Other definitions for escaper that I've seen before include "it's no longer caged" , "One who gets away" , "One runs away" , "Fugitive" , "One making a getaway" .)