Period celebrated by mother and son (4)
I believe the answer is:
xmas
'period' is the definition.
(Xmas is an example)
'by mother and son' is the wordplay.
'by' becomes 'x' (multiplication sign).
'mother' becomes 'ma' (ma can mean mother).
'and' says to put letters next to each other.
'son' becomes 's' (genealogical abbreviation for son).
'x'+'ma'+'s'='XMAS'
'celebrated' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for xmas that I've seen before include "present time" , "December holiday..." , "Christmas (informal)" , "Yuletide" , "Winter festival (informal)" .)