Introduce irrelevant matter of US soldier returning wearing frock! (7)
I believe the answer is:
digress
'introduce irrelevant matter' is the definition.
'us soldier returning wearing frock' is the wordplay.
'us soldier' becomes 'gi' (I've seen this in other clues).
'returning' is a reversal indicator.
'wearing' is an insertion indicator.
'frock' becomes 'dress' ('dress' can be a synonym of 'frock').
'gi' reversed gives 'ig'.
'ig' going inside 'dress' is 'DIGRESS'.
'of' acts as a link.
(Other definitions for digress that I've seen before include "Deviate from the main subject" , "Wander from subject" , "Ramble" , "Stray from main subject" , "Stray from the point" .)