Aptly learn swimming, as father treats child? (10)
I believe the answer is:
paternally
'as father treats child?' is the definition.
'aptly learn swimming' is the wordplay.
'swimming' indicates an anagram.
'aptly'+'learn'='aptlylearn'
'aptlylearn' anagrammed gives 'PATERNALLY'.
(Other definitions for paternally that I've seen before include "In father-like manner" , "Like a father" , "In a fatherly way" .)