Trainers, say, promise to keep mood up (10)
I believe the answer is:
sportswear
'trainers say' is the definition.
The definition suggests a singular noun which matches the answer.
'promise to keep mood up' is the wordplay.
'promise' becomes 'swear' (I've seen this before).
'to' says to put letters next to each other (I've seen this in other clues).
'keep mood up' becomes 'sport' (I can't justify this - if you can you should give a lot more credence to this answer).
'swear' after 'sport' is 'SPORTSWEAR'.
Can you help me to learn more?
(Other definitions for sportswear that I've seen before include "football kit?" , "informal suits" , "clothes for play" , "Clothing" , "Kit for games" .)