Small mail depot to employ husband (6)
I believe the answer is:
spouse
'husband' is the definition.
(a husband is a male spouse)
'small mail depot to employ' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'mail depot' becomes 'po' (I can't justify this - if you can you should believe this answer much more).
'to employ' becomes 'use' (synonyms).
's'+'po'+'use'='SPOUSE'
Can you help me to learn more?
(Other definitions for spouse that I've seen before include "Elizabethan dance" , "Consort" , "old lady?" , "mate" , "Other half" .)