A dwarf's back passage (4)
I believe the answer is:
coda
'passage' is the definition.
(I've seen this before)
'a dwarf's back' is the wordplay.
'dwarf' becomes 'doc' (one of the seven**).
'back' is a reversal indicator.
'a'+'doc'='adoc'
'adoc' backwards is 'CODA'.
(Other definitions for coda that I've seen before include "End piece written for a musical work" , "tail" , "Musical recap" , "End of movement" , "final piece of work" .)