Rex is inside eagerly brewing tea (4,4)
I believe the answer is:
earl grey
'tea' is the definition.
(Earl Grey is a type of tea)
'rex is inside eagerly brewing' is the wordplay.
'rex' becomes 'R' (abbreviation e.g. GR=George Rex, King George).
'is inside' indicates putting letters inside.
'brewing' indicates an anagram.
'eagerly' with letters rearranged gives 'ealgrey'.
'r' inserted within 'ealgrey' is 'EARL GREY'.
(Other definitions for earl grey that I've seen before include "brew" , "Bergamot-flavoured tea blend" , "Type of 22 across" , "Bergamot-flavour tea" , "Tea named after British prime minister" .)