Swimming in her pond is a mood booster (9)
I believe the answer is:
endorphin
'a mood booster' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'swimming in her pond' is the wordplay.
'swimming' indicates anagramming the letters (letters swim around into a new order).
'in'+'her'+'pond'='inherpond'
'inherpond' is an anagram of 'ENDORPHIN'.
'is' is the link.
Can you help me to learn more?
(Other definitions for endorphin that I've seen before include "Hormone in the brain with an analgesic effect" , "Opiate-like substance produced in the brain" , "painkiller" , "Neurochemical with analgesic properties - dinner hop (anag)" .)