Wife loves student's coat (4)
I believe the answer is:
wool
'coat' is the definition.
(I know that wool is a type of coat)
'wife loves student's' is the wordplay.
'wife' becomes 'w' (abbreviation).
'loves' becomes 'oo' (love means 0 in tennis).
'student' becomes 'l' (as in L-plates for learner drivers).
'w'+'oo'+'l'='WOOL'
(Other definitions for wool that I've seen before include "Hair of sheep etc" , "Sheep's clothing" , "Fabric from fleece" , "What can be pulled over eyes to deceive" , "Fabric made from animal hair" .)