Might they possibly restrain racehorses? (8)
I believe the answer is:
trainers
'might they' is the definition.
The definition suggests a plural noun which matches the answer.
'restrain racehorses?' is the wordplay.
'restrain' becomes 'train' (training is a kind of restraining).
'racehorses?' becomes 'ers' (I can't justify this - if you can you should believe this answer much more).
'train'+'ers'='TRAINERS'
'possibly' acts as a link.
Can you help me to learn more?
(Other definitions for trainers that I've seen before include "Sportswear" , "gym shoes" , "Teachers" , "needed to get the most out of racehorses?" , "Coach's" .)