In moat he swims (2,4)
I believe the answer is:
at home
'in' is the definition.
(I've seen this before)
'moat he swims' is the wordplay.
'swims' indicates an anagram.
'moat'+'he'='moathe'
'moathe' anagrammed gives 'AT-HOME'.
(Other definitions for at home that I've seen before include "Oh mate! (anag)" , "Where one lives" , "In (for a party?)" , "Familiar with" , "ready to receive visitors" .)