Form of mature dab seen in outline faintly (9)
I believe the answer is:
adumbrate
'outline faintly' is the definition.
(I've seen this before)
'form of mature dab' is the wordplay.
'form of' indicates an anagram (the letters need to be in another 'form').
'mature'+'dab'='maturedab'
'maturedab' is an anagram of 'ADUMBRATE'.
'seen in' acts as a link.
(Other definitions for adumbrate that I've seen before include "sketch" , "Faintly indicate" , "Depict in outline" , "Show faintly, in outline" , "Foreshadow, be a sign of" .)