Have relative stay quiet (4,3)
I believe the answer is:
keep mum
'stay quiet' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how they can define each other.
'have relative' is the wordplay.
'have' becomes 'keep' (keeping is a kind of having).
'relative' becomes 'mum' (mum is a kind of relative).
'keep'+'mum'='KEEP MUM'
Can you help me to learn more?
(Other definitions for keep mum that I've seen before include "Don't tell a soul" , "Stay silent" , "Remain silent" , "(don't mention it)" .)