Best to be tight-lipped after pot I smashed (7)
I believe the answer is:
optimum
'best' is the definition.
('optimum' can be similar in meaning to 'best')
'tight-lipped after pot i smashed' is the wordplay.
'tight-lipped' becomes 'mum' (I've seen this before).
'after' says to put letters next to each other.
'smashed' indicates an anagram.
'pot'+'i'='poti'
'poti' with letters rearranged gives 'opti'.
'mum' after 'opti' is 'OPTIMUM'.
'to be' is the link.
(Other definitions for optimum that I've seen before include "The most favourable condition" , "Most favourable amount" , "Best (result)" , "top condition" , "model" .)