Person valuing limited company could be auditor's mortgage provider (5)
I believe the answer is:
Here is my best explanation:
'person' is the definition.
(loner is a kind of person)
'auditor's mortgage provider' is the wordplay.
'auditor's' indicates a 'sounds like' (homophone) clue.
'mortgage' becomes 'loan' (I've seen this before).
'provider' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'loan' sounds like 'lone'.
'valuing limited company could be' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for loner that I've seen before include "person, one hating company", "One preferring own company", "Non-conformist", "Enrol (anag)", "Isolated individual", "Person who prefers his own company", "Unsociable person", "Solitary type", "recluse" and "One shunning company".)