Detached settlement's blooming mail! (7)
I believe the answer is:
outpost
'detached settlement's' is the definition.
'outpost' can be an answer for 'settlement's' (outpost is a kind of settlement). I'm not sure about the 'detached' bit.
'blooming mail' is the wordplay.
'blooming' becomes 'out' (plants which have bloomed are 'out').
'mail' becomes 'post' (synonyms).
'out'+'post'='OUTPOST'
Can you help me to learn more?
(Other definitions for outpost that I've seen before include "Distant part (of empire)" , "A distant branch or settlement" , "Distant military camp" , "Small camp some distance away from the main army" , "Remote place" .)