They swim back to have the odd rest (6)
I believe the answer is:
otters
'they swim' is the definition.
Although both the answer and 'swim' are nouns, they are not in the same form of the noun.
'back to have the odd rest' is the wordplay.
'back' says the letters should be written in reverse.
'have' means one lot of letters go next to another.
'the odd' indicates anagramming the letters (I've seen 'the odds' mean this (letters in odd-numbered positions are used)).
'to' reversed gives 'ot'.
'rest' is an anagram of 'ters'.
'ot'+'ters'='OTTERS'
Can you help me to learn more?
(Other definitions for otters that I've seen before include "web-footed creatures" , "Aquatic carnivores" , "Mammals with webbed feet" , "Water animals with webbed feet and brown fur" , "Freshwater carnivorous mammals" .)