Pot of tea, at first, then drank a rum (7)
I believe the answer is:
tankard
'pot' is the definition.
(I've seen this before)
'tea at first then drank a rum' is the wordplay.
'tea' becomes 't' (I've seen this in other clues).
'at first then' says to put letters next to each other.
'rum' indicates anagramming the letters (rum can mean strange or unusual).
'drank'+'a'='dranka'
'dranka' with letters rearranged gives 'ankard'.
't'+'ankard'='TANKARD'
'of' is the link.
(Other definitions for tankard that I've seen before include "Beer-mug" , "Drinker's pot" , "Drank at (anag)" , "One-handled drinking mug" , "Metal beer mug" .)