Injection of wit over island language (7)
I believe the answer is:
punjabi
'language' is the definition.
(I know that Punjabi is a modern language)
'injection of wit over island' is the wordplay.
I cannot quite see how this works, but
'injection' could be 'jab' (I've seen this before) and 'jab' is located in the answer.
'wit' could be 'pun' (pun is a kind of wit) and 'pun' is present in the answer.
'island' could be 'i' (geographical abbreviation) and 'i' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for punjabi that I've seen before include "talk of Amritsar?" , "Indian language" , "From NW India" , "Eastern language" , "From a state in Northwest India" .)