Melissa strangely lacking purpose (7)
I believe the answer is:
aimless
'lacking purpose' is the definition.
(I've seen this before)
'melissa strangely' is the wordplay.
'strangely' indicates anagramming the letters.
'melissa' anagrammed gives 'AIMLESS'.
(Other definitions for aimless that I've seen before include "Melissa (anag) -- unmotivated" , "With no plan or programme" , "Drifting - Melissa (anag)" , "Without purpose or plan" , "Without purpose or target" .)