Surface from swimming extra length's first half (8)
I believe the answer is:
external
'surface' is the definition.
(thesaurus)
'swimming extra length's first half' is the wordplay.
'swimming' is an anagram indicator.
'first half' indicates taking half.
'length' with half the letters taken is 'len'.
'extra'+'len'='extralen'
'extralen' with letters rearranged gives 'EXTERNAL'.
'from' is the link.
(Other definitions for external that I've seen before include "Surface" , "involving foreign nations" , "abroad" , "Outer" , "Lying outside" .)