He joins in the row (4)
I believe the answer is:
tier
'row' is the definition.
(a tier is a row of things)
'he joins in the' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
The remaining letters 'ier' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tier that I've seen before include "He might do sailor's knots" , "'Level, grade (4)'" , "Rank - order" , "Wedding cake layer" , "Row, level" .)