Look to return in army post where duty is relaxed (4,4)

'where duty is relaxed' is the definition.
(where goods are exempt from customs duties)

'look to return in army post' is the wordplay.
'look' becomes 'peer' (peering is a kind of looking).
'to return' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'army post' becomes 'fort' (a fort is a military post).
'peer' written backwards gives 'reep'.
'reep' inserted into 'fort' is 'FREE PORT'.