Desire shown by one bather, primarily to get into pool (6)
I believe the answer is:
libido
'desire' is the definition.
(libido is a kind of desire)
'one bather primarily to get into pool' is the wordplay.
'one' becomes 'i' (Roman numeral).
'primarily' suggests taking the first letters.
'to get into' indicates putting letters inside.
'pool' becomes 'lido' (I've seen this before).
The initial letter of 'bather' is 'b'.
'i'+'b'='ib'
'ib' inserted inside 'lido' is 'LIBIDO'.
'shown by' is the link.
(Other definitions for libido that I've seen before include "Emotional energy associated with sexual desire" , "Freud's sex drive" , "Sexual drive" , "Sexual impulse" , "Sexual or psychological urge" .)