Mature mother cradling son in Michelangelo's work? (3,6)
I believe the answer is:
old master
'michelangelo's work?' is the definition.
'mature mother cradling son' is the wordplay.
'mature' becomes 'old' (associated in meaning).
'mother' becomes 'mater' (old-fashioned term for mother).
'cradling' means one lot of letters goes inside another.
'son' becomes 's' (genealogical abbreviation for son).
'mater' going around 's' is 'master'.
'old'+'master'='OLD MASTER'
'in' is the link.
(Other definitions for old master that I've seen before include "Great painting of former times" , "He was a great exponent of art" , "Liege lord" , "A great European painter - a painting by one" , "scholar with greybeard" .)