Weak and ultimately disappointing cup of tea (5)
I believe the answer is:
thing
'cup of tea' is the definition.
(as in 'that's not quite my cup of tea')
'weak and ultimately disappointing' is the wordplay.
'weak' becomes 'thin' (similar in meaning).
'and' says to put letters next to each other.
'ultimately' says to take the final letters.
The final letter of 'disappointing' is 'g'.
'thin'+'g'='THING'
(Other definitions for thing that I've seen before include "Object, artefact" , "Article" , "Inanimate object" , "A creature" , "Affair - matter" .)