Doctor joins a mother in play (5)
I believe the answer is:
drama
'play' is the definition.
(I know that play can be written as drama)
'doctor joins a mother' is the wordplay.
'doctor' becomes 'Dr' (abbreviation).
'joins' is a charade indicator (letters next to each other).
'mother' becomes 'ma' (Ma is another word for mother or mum).
'dr'+'a'+'ma'='DRAMA'
'in' is the link.
(Other definitions for drama that I've seen before include "Theatrical work" , "Series of events involving conflicting forces" , "Play - excitement" , "Theatrical piece" , "High emotion" .)