Wife rings trainee for yarn (4)
I believe the answer is:
wool
'for yarn' is the definition.
(I've seen this before)
'wife rings trainee' is the wordplay.
'wife' becomes 'w' (abbreviation).
'rings' becomes 'oo' (multiple circles).
'trainee' becomes 'l' (as in an L-plate for a learner driver).
'w'+'oo'+'l'='WOOL'
(Other definitions for wool that I've seen before include "Sheep's yarn" , "It could become felt" , "Sheep coat" , "Yarn spun" , "Yarn - cloth" .)