Stop at sea: the Spanish brought into harbour (5)
I believe the answer is:
belay
'stop at sea' is the definition.
(nautical term for 'stop')
'the spanish brought into harbour' is the wordplay.
'the spanish' becomes 'el'.
'brought into' is an insertion indicator.
'harbour' becomes 'bay'.
'el' placed within 'bay' is 'BELAY'.
(Other definitions for belay that I've seen before include "Turn a rope round something to secure it" , "Stop" , "Make fast" .)