With a tench? nothing unexpected but charm! (7)
I believe the answer is:
enchant
'charm' is the definition.
(enchanting is a kind of charming)
'with a tench? nothing unexpected' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'but' acts as a link.
Can you help me to learn more?
(Other definitions for enchant that I've seen before include "Captivate, charm" , "Appeal" , "Entrance" , "Fascinate - delight" , "Bewitch, charm" .)