Tartan Army’s leader wearing top to support Portugal (5)
I believe the answer is:
plaid
'tartan' is the definition.
(I know that tartan can be written as plaid)
'army's leader wearing top to support portugal' is the wordplay.
'leader' indicates taking the first letters.
'wearing' means one lot of letters goes inside another.
'top' becomes 'lid' (lid is a kind of top).
'to support' means one lot of letters go next to another (in a down clue, some letters hold others up).
'portugal' becomes 'p' (common abbreviation eg on car number plates).
The initial letter of 'armys' is 'a'.
'a' put into 'lid' is 'laid'.
'laid' put after 'p' is 'PLAID'.
(Other definitions for plaid that I've seen before include "Tartan-pattern cloth" , "Part of Scots dress or Welsh party" , "Cloth for a Highlander" , "piece of cloth" , "Scots cloth - Welsh party" .)