Wife rings student to get yarn (4)
I believe the answer is:
wool
'yarn' is the definition.
(I know that wool is a type of yarn)
'wife rings student' is the wordplay.
'wife' becomes 'w' (abbreviation).
'rings' becomes 'oo' (two O's - O resembles a ring).
'student' becomes 'l' (as in L-plates for learner drivers).
'w'+'oo'+'l'='WOOL'
'to get' acts as a link.
(Other definitions for wool that I've seen before include "Yarn spun" , "Yarn - cloth" , "Fabric made from the hair of sheep" , "What can be pulled over eyes to deceive" , "It could become felt" .)