He can't succeed by himself (6)
I believe the answer is:
coheir
'he can't succeed' is the definition.
The definition suggests a singular noun which matches the answer.
'himself' is the wordplay.
I cannot really understand how this works, but
'himself' could be 'he' (I've seen this in other clues) and 'he' is found in the answer.
The remaining letters 'coir' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'by' is the link.
Can you help me to learn more?
(Other definitions for coheir that I've seen before include "Not the only one to succeed" , "one of several in succession?" .)