Back trouble unknown in diminutive Jedi master (4)
I believe the answer is:
yoda
'diminutive jedi master' is the definition.
(I've seen this in another clue)
'back trouble unknown' is the wordplay.
'back' says the letters should be written in reverse.
'trouble' becomes 'ado' (both can mean bother or commotion).
'unknown' becomes 'y' (algebra).
'ado'+'y'='adoy'
'adoy' backwards is 'YODA'.
'in' is the link.
(Other definitions for yoda that I've seen before include "one with idiosyncratic syntax" , "Grand Master of the Jedi Order" , "Green being" .)