A base for thieves maybe in port (4)
I believe the answer is:
aden
'port' is the definition.
(port in Yemen)
'a base for thieves maybe in' is the wordplay.
'a' becomes 'an'.
'base' becomes 'e' (mathematical symbol).
'for' says to put letters next to each other (I've seen this in other clues).
'thieves maybe' becomes 'd' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'e' put after 'd' is 'de'.
'an' placed around 'de' is 'ADEN'.
Can you help me to learn more?
(Other definitions for aden that I've seen before include "Yemen's chief port" , "foreign climes" , "Former Br. Gulf colony" , "Dean is in the Yemen, strangely" , "Yemeni gulf" .)