Initially en route today to block Spanish port, causing complaint (7)
I believe the answer is:
vertigo
'complaint' is the definition.
(medical complaint)
'initially en route today to block spanish port' is the wordplay.
'initially' indicates taking the first letters.
'to block' indicates putting letters inside (block can mean to fill a hole in something).
'spanish port' becomes 'vigo' (port in Spain).
The first letters of 'en route today' is 'ert'.
'ert' put within 'vigo' is 'VERTIGO'.
'causing' is the link.
(Other definitions for vertigo that I've seen before include "Dizzy feeling when at great height" , "Hitchcock film - giddiness" , "Dizziness due to disorder in sense of balance" , "unbalanced feeling" , "Sensation brought on by great heights" .)