Subway adjust large sandwiches back in kitchen (6)
I believe the answer is:
tunnel
'subway' is the definition.
(thesaurus)
'adjust large sandwiches back in kitchen' is the wordplay.
'adjust' becomes 'tune' (tuning is a kind of adjusting).
'large' becomes 'L' (eg in clothes sizes).
'sandwiches' means one lot of letters goes inside another (to sandwich is to block something in).
'back in' indicates one should take the final letters (the 'back' of the word).
The final letter of 'kitchen' is 'n'.
'tune'+'l'='tunel'
'tunel' going around 'n' is 'TUNNEL'.
(Other definitions for tunnel that I've seen before include "See 7" , "See 1" , "Excavated passage" , "See 11" , "An underground route" .)