Setback star encountered at first, finding plumber’s tool (8)
I believe the answer is:
blowpipe
'plumber's tool' is the definition.
(blowpipe is a tool in plumbing)
'setback star encountered at first' is the wordplay.
'setback' becomes 'blow' (blow can mean a setback or difficulty).
'star' becomes 'pip' (star on a military uniform).
'at first' says to take the initial letters.
The first letter of 'encountered' is 'e'.
'blow'+'pip'+'e'='BLOWPIPE'
'finding' is the link.
(Other definitions for blowpipe that I've seen before include "Native dart weapon" , "Use it to shape glass" , "Tubular weapon" , "Dart launcher" , "Primitive weapon held in the mouth" .)