Scottish cap (3)
I believe the answer is:
'scottish cap' is the definition.
(I've seen this before)
This is all the 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)'" .)