Indian perhaps in old vessel crossing Spain (5)
I believe the answer is:
ocean
'indian perhaps' is the definition.
(Indian Ocean is an example)
'old vessel crossing spain' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'vessel' becomes 'can' (I've seen this before).
'crossing' indicates putting letters inside.
'spain' becomes 'e' (abbreviation for Espana).
'can' placed around 'e' is 'cean'.
'o'+'cean'='OCEAN'
'in' acts as a link.
(Other definitions for ocean that I've seen before include "Salt water expanse" , "Huge expanse" , "Large expanse of sea" , "Vast quantity of liquid" , "Great sea" .)