Are useful to an animal when it's swimming (6)
I believe the answer is:
assist
'it's swimming' is the definition.
I can't tell whether this defines the answer.
'are useful to an animal' is the wordplay.
'are useful' becomes 'ist' (I can't explain this - if you can you should give a lot more credence to this answer).
'to' says to put letters next to each other (I've seen this in other clues).
'an animal' becomes 'ass' (term for a donkey).
'ist' put after 'ass' is 'ASSIST'.
'when' acts as a link.
Can you help me to learn more?
(Other definitions for assist that I've seen before include "Pass to scorer" , "Expedite" , "Be helpful to" , "providing help?" , "Help member" .)