Snail messes around, showing a lack of purpose (11)
I believe the answer is:
aimlessness
'a lack of purpose' is the definition.
(I've seen this before)
'snail messes around' is the wordplay.
'around' indicates anagramming the letters.
'snail'+'messes'='snailmesses'
'snailmesses' with letters rearranged gives 'AIMLESSNESS'.
'showing' acts as a link.
(Other definitions for aimlessness that I've seen before include "Futility" , "Lack of purpose" .)