Watch delivered extreme reliability (6)
I believe the answer is:
sentry
'watch' is the definition.
(synonyms)
'delivered extreme reliability' is the wordplay.
'delivered' becomes 'sent' (I've seen this in another clue).
'extreme' says to hollow out the word (remove centre letters).
'reliability' with its middle removed is 'ry'.
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "One keeps guard" , "Military guard" , "Picket" , "Guard on watch" , "Look-out, guard" .)