Eccentric who keeps everything in sacks (6)
I believe the answer is:
whacko
'eccentric who keeps everything in sacks' is the definition.
'whacko' can be an answer for 'eccentric' (I've seen this before). I'm not certain of the rest of the definition.
'eccentric who keeps' is the wordplay.
'eccentric' becomes 'ack' (I can't explain this - if you can you should believe this answer much more).
'keeps' is an insertion indicator (inserted letters are kept inside).
'ack' placed into 'who' is 'WHACKO'.
Can you help me to learn more?
(Other definitions for whacko that I've seen before include "Mad" , "Eccentric" , "Jolly good" , "nuts" .)