Not exactly a tin, but a container of tuna (3)
I believe the answer is:
tun
'container of tuna' is the definition.
'tun' can be an answer for 'container' (tun is a kind of container). I am not certain of the 'of tuna' bit.
'not exactly a tin but a' is the wordplay.
I cannot quite see how this works, but
'not' could be 'un' (prefix meaning 'not') and 'un' is found within the answer.
A single letter 't' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for tun that I've seen before include "Large cask (holding 252 gallons)" , "Barrel holding many gallons" , "butt" , "Large cask for liquid" , "Container of beer" .)