A sub, reportedly unescorted (5)
I believe the answer is:
alone
'unescorted' is the definition.
(not escorted by other people)
'a sub reportedly' is the wordplay.
'sub' becomes 'loan' (British term for advance against income).
'reportedly' indicates a 'sounds like' (homophone) clue.
'loan' sounds like 'lone'.
'a'+'lone'='ALONE'
(Other definitions for alone that I've seen before include "Singly" , "Note of charges" , "Without friends" , "Solo" , "Without others present" .)