Cover up mother’s knee at first (4)
I believe the answer is:
mask
'cover up' is the definition.
(thesaurus)
'mother's knee at first' is the wordplay.
'mother's' becomes 'mas' (ma is a kind of mother).
'at first' suggests taking the first letters.
The first letter of 'knee' is 'k'.
'mas'+'k'='MASK'
(Other definitions for mask that I've seen before include "Camouflage" , "One to hide face" , "Face covering; disguise" , "Face-cover" , "Visor; disguise" .)