Swimming team's mostly neat, languid swimmers (8)
I believe the answer is:
manatees
'languid swimmers' is the definition.
Although both the answer and definition are plural nouns, I can't see how they can define each other.
'swimming team's mostly neat' is the wordplay.
'swimming' indicates anagramming the letters (letters swim around into a new order).
'mostly' means to remove the first letter.
'teams' with its first letter taken away is 'eams'.
'eams'+'neat'='eamsneat'
'eamsneat' is an anagram of 'MANATEES'.
Can you help me to learn more?
(Other definitions for manatees that I've seen before include "Large plant-eating mammals of tropical waters" , "Sirenians" , "Water creatures" , "Men at sea (anag) - aquatic mammals" .)