As part of agreement demand ridiculous platitudes lacking depth (9)
I believe the answer is:
stipulate
'as part of agreement demand' is the definition.
(to stipulate is to demand during a negotiation)
'ridiculous platitudes lacking depth' is the wordplay.
'ridiculous' is an anagram indicator.
'lacking' is a deletion indicator.
'depth' becomes 'd' (abbreviation in geometry).
'platitudes' with 'd' taken away is 'platitues'.
'platitues' with letters rearranged gives 'STIPULATE'.
(Other definitions for stipulate that I've seen before include "Define, require" , "Demand as part of agreement" , "Demand, specify" , "Specify as a condition or requirement" , "Make a demand" .)