In port, the authority I had for tartan (5)
I believe the answer is:
plaid
'for tartan' is the definition.
(synonyms)
'in port the authority i' is the wordplay.
'in' means one lot of letters goes inside another.
'port' becomes 'p'.
'the authority' becomes 'lad' (I can't justify this - if you can you should believe this answer much more).
'p'+'lad'='plad'
'plad' enclosing 'i' is 'PLAID'.
'had' acts as a link.
Can you help me to learn more?
(Other definitions for plaid that I've seen before include "Scots cloth - Welsh party" , "Tartan twilled cloth" , "Tartan patterned cloth" , "woollen cloth" , "Cloth with tartan pattern" .)