Impressive feat by service lines (7)
I believe the answer is:
couplet
'lines' is the definition.
(I've seen this before)
'impressive feat by service' is the wordplay.
'impressive feat' becomes 'coup' (coup can mean a triumphant feat).
'by' means one lot of letters go next to another.
'service' becomes 'let' (type of tennis service).
'coup'+'let'='COUPLET'
(Other definitions for couplet that I've seen before include "Pair of verse lines" , "bit of poetry" , "piece of poetry" , "Two lines of verse, often rhyming" , "Two successive lines of verse, usually rhyming" .)