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' is a reversal indicator.
'have' says to put letters next to each other.
'the odd' indicates an anagram (I've seen 'the odds' mean this (letters in odd-numbered positions are used)).
'to' written backwards gives 'ot'.
'rest' anagrammed gives '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" .)