Colony on strike mail (7)
I believe the answer is:
outpost
'colony' is the definition.
(I know that outpost is a type of colony)
'on strike mail' is the wordplay.
'on strike' becomes 'out' (I've seen this before).
'mail' becomes 'post' (synonyms).
'out'+'post'='OUTPOST'
(Other definitions for outpost that I've seen before include "Remote part of an empire" , "Distant part (of empire)" , "Remote military site" , "Remote settlement" , "isolated settlement" .)