Lack of purpose? Not so, when embraced by heads (13)
I believe the answer is:
pointlessness
'lack of purpose? not so when embraced by heads' is the definition.
I can't judge whether this definition defines the answer.
'lack of purpose? not so when embraced by' is the wordplay.
'lack of' means to remove the last letter (I've seen 'lacking' mean this (the word is incomplete or lacking)).
'purpose?' becomes 'point' (I've seen this before).
'not so' becomes 'less' (eg 'not so big' means 'less big').
'when embraced by' becomes 'sness' (I can't explain this - if you can you should give a lot more credence to this answer).
'point'+'less'='pointless'
'pointless' with its final letter removed is 'pointles'.
'pointles'+'sness'='POINTLESSNESS'
Can you help me to learn more?
(Other definitions for pointlessness that I've seen before include "that is demonstrated by blunt type" , "Futility" .)