For tea, initially, use a mug that's full scale (5)
I believe the answer is:
gamut
'full scale' is the definition.
'gamut' can be an answer for 'scale' (I've seen this before). I'm unsure of the 'full' bit.
'for tea initially use a mug' is the wordplay.
'for tea' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'initially' is a charade indicator (letters next to each other).
'use' indicates an anagram.
'a'+'mug'='amug'
'amug' anagrammed gives 'gamu'.
't' put after 'gamu' is 'GAMUT'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for gamut that I've seen before include "Full (emotional) range" , "Complete range or scale" , "Ambit" , "The whole range or scope of anything" , "Whole range (from A to Z)" .)