Falsely smile as lacking purpose (7)
I believe the answer is:
aimless
'lacking purpose' is the definition.
(I've seen this in another clue)
'falsely smile as' is the wordplay.
'falsely' indicates anagramming the letters.
'smile'+'as'='smileas'
'smileas' is an anagram of 'AIMLESS'.
(Other definitions for aimless that I've seen before include "Without purpose or plan" , "at a loose end" , "Drifting - Melissa (anag)" , "Without direction or ambition" , "Lacking a goal" .)