Self-control of Government department on European limit (10)
I believe the answer is:
moderation
'self-control' is the definition.
(I've seen this before)
'government department on european limit' is the wordplay.
'government department' becomes 'mod' (Ministry of Defence).
'on' means one lot of letters go next to another.
'european' becomes 'e' (abbreviation e.g. EU).
'limit' becomes 'ration' (rationing is a kind of limiting).
'mod'+'e'+'ration'='MODERATION'
'of' acts as a link.
(Other definitions for moderation that I've seen before include "restraint" , "avoids extremes" , "Avoidance of excess" , "Avoidance of extremes" , "self-control" .)