Unaided by a sub, by all accounts (5)
I believe the answer is:
alone
'unaided' is the definition.
(I've seen this in another clue)
'by a sub by all accounts' is the wordplay.
'by' means one lot of letters go next to another.
'sub' becomes 'loan' (British term for advance against income).
'by all accounts' indicates a 'sounds like' (homophone) clue ('by all accounts' can mean 'we hear').
'loan' sounds like 'lone'.
'a'+'lone' is 'ALONE'.
(Other definitions for alone that I've seen before include "unique" , "Solo" , "without another" , "Without anybody or anything else" , "On one's own" .)