Smile as ridiculously lacking in purpose (7)
I believe the answer is:
aimless
'lacking in purpose' is the definition.
(without aim or purpose)
'smile as ridiculously' is the wordplay.
'ridiculously' is an anagram indicator.
'smile'+'as'='smileas'
'smileas' anagrammed gives 'AIMLESS'.
(Other definitions for aimless that I've seen before include "Without purpose or direction" , "Drifting with no purpose" , "Lacking a goal" , "Without direction or ambition" , "Without purpose or target" .)