Seabird messing up girl's head in Yorkshire port (7,4)
I believe the answer is:
herring gull
'seabird' is the definition.
(herring gull is a kind of seabird)
'messing up girl's head in yorkshire port' is the wordplay.
'messing up' becomes 'erring'.
'head' indicates taking the first letters.
'in' indicates putting letters inside.
'yorkshire port' becomes 'hull'.
The initial letter of 'girl' is 'g'.
'erring'+'g'='erringg'
'erringg' inserted within 'hull' is 'HERRING-GULL'.
(Other definitions for herring gull that I've seen before include "Fish-eating sea bird" , "Large common seabird" , "Common sea bird" .)