Racecourse gets sailors' purgative (5,5)
I believe the answer is:
epsom salts
'purgative' is the definition.
(I've seen this before)
'racecourse gets sailors'' is the wordplay.
'racecourse' becomes 'epsom' (I've seen this before).
'gets' means one lot of letters go next to another.
'sailors'' becomes 'salts' (salt is a kind of sailor).
'epsom'+'salts'='EPSOM SALTS'
(Other definitions for epsom salts that I've seen before include "a purge" , "Medicinal preparation" , "Purgative" , "They should clear out!" , "Medicine taken for heartburn and constipation" .)