In an unmentionable way? (7)
I believe the answer is:
tacitly
'in' is the definition.
Both the definition and answer are adverbs. Maybe you can see an association between them that I can't see?
'an unmentionable way?' is the wordplay.
I cannot quite see how this works, but
'an' could be 'i' (Roman numeral for one) and 'i' is found within the answer.
'way?' could be 'act' (way is a kind of act) and 'act' is found in the remaining letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tacitly that I've seen before include "Without a word" , "By unexpressed agreement" , "understandably" , "By implication" .)