Middle part of three inches, perhaps, is so long (4-4)
I believe the answer is:
chin-chin
'is so long' is the definition.
(I've seen this before)
'middle part of three inches' is the wordplay.
I cannot really see how this works, but
'part' could be 'inch' (I have seen 'Part of foot ' mean 'inch' so perhaps 'part' could also mean 'inch') and 'inch' is present in the answer.
The remaining letters 'chin' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'perhaps' acts as a link.
Can you help me to learn more?
(Other definitions for chin-chin that I've seen before include "Down the hatch!" , "Cheers, bottoms up" , "good health ..." .)