Determined about reforming soonest? Not very (3,2)
I believe the answer is:
set on
'determined' is the definition.
(I've seen this before)
'reforming soonest? not very' is the wordplay.
'reforming' indicates anagramming the letters.
'not' is a deletion indicator.
'very' becomes 'so' (I've seen this in other clues).
'soonest' with 'so' removed is 'onest'.
'onest' anagrammed gives 'SET ON'.
'about' is the link.
(Other definitions for set on that I've seen before include "Determined" , "Attack" , "Go" .)