Mum will pose holding the boy (6)
I believe the answer is:
silent
'mum' is the definition.
(to keep mum can informally mean to stay silent)
'pose holding the boy' is the wordplay.
'pose' becomes 'sit' (synonyms).
'holding' means one lot of letters goes inside another.
'the boy' becomes 'len' (boy's name).
'sit' enclosing 'len' is 'SILENT'.
'will' acts as a link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for silent that I've seen before include "Quiet -- 13 (anag)" , "Free from noise" , "Enlist (anag) - mum" , "still" , "A continent" .)