Strange case -- youth is one who's persuaded without effort (4,5)
I believe the answer is:
easy touch
'one who's persuaded without effort' is the definition.
(someone who does something when asked)
'strange case youth' is the wordplay.
'strange' indicates an anagram.
'case'+'youth'='caseyouth'
'caseyouth' anagrammed gives 'EASY TOUCH'.
'is' is the link.
(Other definitions for easy touch that I've seen before include "Someone readily manipulated" , "Soft-hearted, not hard to borrow from" , "Person who lends money freely - simply tangible" , "doormat?" , "Say touche, being an over-generous type" .)