Those people joining cricket side stop improving (2,3,4)
I believe the answer is:
on the mend
'improving' is the definition.
(on the mend means improving after an illness)
'those people joining cricket side stop' is the wordplay.
'those people' becomes 'them' ('they' refers to a group of people).
'joining' says to put letters next to each other.
'cricket side' becomes 'on' (cricket term, on or leg side).
'stop' becomes 'end' (synonyms).
'them' after 'on' is 'onthem'.
'onthem'+'end'='ON THE MEND'
(Other definitions for on the mend that I've seen before include "Regaining health" , "convalescing" , "undergoing recovery?" , "Informally getting better, improving after illness or accident" , "Making recovery" .)