Again advanced smaller sum to head, showing persistence (14)
I believe the answer is:
Here is my best explanation:
'persistence' is the definition.
'again advanced smaller sum to head' is the wordplay.
'again' becomes 're'.
'advanced' becomes 'lent' (money).
'smaller sum' becomes 'less'.
'to head' becomes 'ness' (ness is a headland).
'showing' is the link.
(Another definition for relentlessness that I've seen is "show persistence".)