Maybe trainers are foul-mouthed keeping tantrum up (10)
I believe the answer is:
sportswear
'maybe trainers' is the definition.
(I know that sneakers is a type of athletic shoe)
'foul-mouthed keeping tantrum up' is the wordplay.
'foul-mouthed' becomes 'swear' (I can't explain this - if you can you should believe this answer much more).
'keeping' indicates putting letters inside (some letters kept inside).
'tantrum' becomes 'strop' (strop can mean a temper tantrum).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'strop' backwards is 'ports'.
'swear' going around 'ports' is 'SPORTSWEAR'.
'are' is the link.
Can you help me to learn more?
(Other definitions for sportswear that I've seen before include "informal suits" , "football kit?" , "in which triathletes swim" , "Clothing" , "clothes for play" .)