Cap half on in swimming pool (4)
I believe the answer is:
lido
'swimming pool' is the definition.
(lido is a type of open-air swimming pool)
'cap half on' is the wordplay.
'cap' becomes 'lid' (eg the lid of a bottle).
'half' suggests halving the letters.
'on' halved is 'o'.
'lid'+'o'='LIDO'
'in' acts as a link.
(Other definitions for lido that I've seen before include "bathing-beach" , "bathing facilities" , "Swimming bath" , "Venetian resort" , "Public open swimming pool" .)