Fine under hat, one invisible (6)
I believe the answer is:
subtle
'fine' is the definition.
(synonyms)
'under hat one invisible' is the wordplay.
I cannot really understand how this works, but
'under' could be 'sub' (I've seen this before) and 'sub' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for subtle that I've seen before include "Difficult to analyse; understated" , "'Fine, elusive (6)'" , "Ingenious, difficult to analyse" , "Delicate - crafty - indirect" , "Discreet" .)