Promise to protect harbours in Nike's domain (10)
I believe the answer is:
sportswear
'nike's domain' is the definition.
The definition suggests a singular noun which matches the answer.
'promise to protect harbours' is the wordplay.
'promise' becomes 'swear' (as in swearing something is true).
'to protect' means one lot of letters goes inside another (inserted letters are held or protected).
'harbours' becomes 'ports' (I've seen this before).
'swear' enclosing 'ports' is 'SPORTSWEAR'.
'in' is the link.
Can you help me to learn more?
(Other definitions for sportswear that I've seen before include "football kit?" , "Gear" , "in which triathletes swim" , "clothes for play" , "Casual clothes" .)