Some Dublin troops opening passage (5)
I believe the answer is:
intro
'opening passage' is the definition.
(I've seen this before)
'some dublin troops' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'INTRO' is hidden within 'dublin troops'.
(Other definitions for intro that I've seen before include "Prelude" , "Start of work" , "Song's first few bars" , "Opening passage (of music) (colloq.)" , "Preamble" .)