Subtle implication but a patently obvious one (8)
I believe the answer is:
overtone
'subtle implication' is the definition.
'overtone' can be an answer for 'implication' (I've seen this in another clue). I'm not certain of the 'subtle' bit.
'a patently obvious one' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'one' and 'one' is found in the answer.
'obvious' could be 'overt' (similar in meaning) and 'overt' is present in the answer.
No letters remain.
This explanation may well be incorrect...
'but' is the link.
Can you help me to learn more?
(Other definitions for overtone that I've seen before include "Suggestion" , "hidden meaning" , "Subtle quality" , "Subtle implication or connotation" , "Innuendo perhaps" .)