Grid in FT cryptic lacking purpose? (8)
I believe the answer is:
drifting
'lacking purpose?' is the definition.
(drifting around aimlessly)
'grid in ft cryptic' is the wordplay.
'cryptic' indicates an anagram (a cryptic or obscure order of the letters).
'grid'+'in'+'ft'='gridinft'
'gridinft' with letters rearranged gives 'DRIFTING'.
(Other definitions for drifting that I've seen before include "Floating with the current" , "Floating about aimlessly" , "Aimlessly wandering from place to place" .)