Sports star may get this £1000 bit of activewear in three sizes (5,4)
I believe the answer is:
grand slam
'sports star may get this ps1000 bit of activewear' is the definition.
The definition suggests a singular noun which matches the answer.
'in three sizes' is the wordplay.
'three sizes' can mean three replacements for 'size'.
'in' is an insertion indicator.
'size' becomes 'grandam' (I can't justify this - if you can you should give a lot more credence to this answer).
'size' becomes 'S' (small clothing size).
'size' becomes 'L' (large clothing size).
's'+'l'='sl'
'grandam' placed around 'sl' is 'GRAND SLAM'.
Can you help me to learn more?
(Other definitions for grand slam that I've seen before include "Feat of winning all the tricks in a hand of bridge" , "Winning of all of a group of tournaments in tennis" , "Winning of all tricks (or major tournaments)" , "Sporting achievement" , "Top bid in bridge" .)