Old second line heading for Omani capital (4)
I believe the answer is:
oslo
'capital' is the definition.
(capital of Norway)
'old second line heading for omani' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'second' becomes 's' (common abbreviation).
'line' becomes 'l' (used when specifying particular lines from a poem).
'heading for' indicates taking the first letters.
The initial letter of 'omani' is 'o'.
'o'+'s'+'l'+'o'='OSLO'
(Other definitions for oslo that I've seen before include "foreign port" , "European capital city (found in Czechoslovakia!)" , "Norwegian chief port" , "City in Norway" , "European capital in go-slow" .)