Foreign capital excursion on oil rig, occasionally (7)
I believe the answer is:
tripoli
'foreign capital' is the definition.
(capital of Libya)
'excursion on oil rig occasionally' is the wordplay.
'excursion' becomes 'trip' (I've seen this before).
'on' says to put letters next to each other.
'occasionally' means one should take alternating letters.
'oil'+'rig'='oilrig'
The alternate letters of 'oilrig' are 'oli'.
'trip'+'oli'='TRIPOLI'
(Other definitions for tripoli that I've seen before include "North African port" , "Libyan port" , "North African city" , "powdery deposit" , "North African capital" .)