Briefly test coupling that holds new rotating pin (8)
I believe the answer is:
trunnion
'rotating pin' is the definition.
(machine part)
'briefly test coupling that holds new' is the wordplay.
'briefly' means to remove the last letter.
'test' becomes 'try' (synonyms).
'coupling' becomes 'union' (synonyms).
'that holds' means one lot of letters goes inside another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'try' with its last letter taken off is 'tr'.
'union' enclosing 'n' is 'unnion'.
'tr'+'unnion'='TRUNNION'
(Other definitions for trunnion that I've seen before include "Turn in on (anag)" , "part of a cannon" , "support - or part of it?" , "One of pair of supports on which something pivots" , "Supporter" .)