Say it's an advantage right through (6)
I believe the answer is:
assert
'say' is the definition.
(I know that say is a kind of assert)
'an advantage right through' is the wordplay.
I cannot quite see how this works, but
'an' could be 'a' and 'a' is found in the answer.
'right' could be 'rt' (abbreviation as in Rt Hon- the Right Honourable) and 'rt' is found within the answer.
The remaining letters 'sse' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for assert that I've seen before include "Declare, state vigorously" , "Express forcibly" , "State firmly and positively" , "Declare, profees" , "Declare, claim" .)