Fine head — using it? (8)
I believe the answer is:
thinking
'it?' is the definition.
I can't tell whether this defines the answer.
'fine head' is the wordplay.
'fine' becomes 'thin' ('thin' can be similar in meaning to 'fine').
'head' becomes 'king' (**).
'thin'+'king'='THINKING'
'using' acts as a link.
(Other definitions for thinking that I've seen before include "rationale" , "Thought" , "Fancying" , "giving opinion" , "Take" .)