All there is on growth stimulant plugged by guys (6,6)
I believe the answer is:
compos mentis
'all there' is the definition.
(both can mean mentally sound)
'is on growth stimulant plugged by guys' is the wordplay.
'on' says to put letters next to each other.
'growth stimulant' becomes 'compost' (I can't justify this - if you can you should give a lot more credence to this answer).
'plugged by' is an insertion indicator (plug can mean to block up a gap).
'guys' becomes 'men' (guy can mean a man).
'compost' placed around 'men' is 'composment'.
'is' put after 'composment' is 'COMPOS MENTIS'.
Can you help me to learn more?
(Other definitions for compos mentis that I've seen before include "Sane" , "Of sound mind (Latin)" , "Capable of reasoning" .)