Swimming pools in solid form (5)
I believe the answer is:
lidos
'swimming pools' is the definition.
(I've seen this before)
'solid form' is the wordplay.
'form' is an anagram indicator (a different form of the same letters).
'solid' anagrammed gives 'LIDOS'.
'in' is the link.
(Other definitions for lidos that I've seen before include "leisure facilities" , "Open-air swimming pools" , "Open air swimming pools" , "swimmers like them" , "bathing beaches" .)