A smile's terribly haphazard (7)
I believe the answer is:
aimless
'haphazard' is the definition.
(I've seen this before)
'a smile's terribly' is the wordplay.
'terribly' indicates an anagram.
'smiles' anagrammed gives 'imless'.
'a'+'imless'='AIMLESS'
(Other definitions for aimless that I've seen before include "Without purpose or direction" , "Without purpose or target" , "Without purpose, like Melissa" , "Like characters in 5D {WAITING FOR GODOT]" , "Lacking ambition or target" .)