Postmen I am rounding up for Christmas shows (10)
I believe the answer is:
pantomimes
'christmas shows' is the definition.
(I've seen this before)
'postmen i am rounding up' is the wordplay.
'rounding up' is an anagram indicator.
'postmen'+'i'+'am'='postmeniam'
'postmeniam' with letters rearranged gives 'PANTOMIMES'.
'for' is the link.
(Other definitions for pantomimes that I've seen before include "Christmas shows" , "Dick Whittington etc" .)