You can tell it's just a yarn (4)
I believe the answer is:
tale
'a yarn' is the definition.
('tale' can be a synonym of 'yarn')
'you can tell it's just' is the wordplay.
I cannot quite see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
The remaining letters 'ale' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tale that I've seen before include "Report" , "See 12" , "Late with a story" , "Old wives' recital?" , "Narrative - fabrication" .)