Scottish cap (3)
I believe the answer is:
'scottish cap' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tam that I've seen before include "Ratings compiled on TV listener numbers" , "TV audience ratings" , "...-o'-shanter, Scottish cap" , "Turn over the mat for a cap" , "'Scottish cloth cap, in short (3)'" .)