Support mum during visit (8)
I believe the answer is:
mainstay
'support' is the definition.
(mainstay is a kind of support)
'mum during visit' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'during' becomes 'in' (e.g. during February means in February).
'visit' becomes 'stay' (I've seen this in another clue).
'ma'+'in'+'stay'='MAINSTAY'
(Other definitions for mainstay that I've seen before include "Linchpin, principle support" , "Strongest support" , "Bulwark" , "Most important support" , "Rope to the mast is the chief support" .)