Cautious about tea mostly over-diluted (6)
I believe the answer is:
watery
'diluted' is the definition.
(similar in meaning)
'cautious about tea mostly' is the wordplay.
'cautious' becomes 'wary' (similar in meaning).
'about' means one lot of letters goes inside another.
'mostly' means to remove the last letter.
'tea' with its last letter taken away is 'te'.
'wary' placed around 'te' is 'WATERY'.
'over' acts as a link.
(Other definitions for watery that I've seen before include "Weak, too diluted" , "(Of food) thin, tasteless" , "Insipid" , "Aqueous" , "Overly diluted, thin" .)