Clearly there's a port authority in outer Londonderry (7)
I believe the answer is:
plainly
'clearly' is the definition.
('plainly' can be a synonym of 'clearly')
'a port authority in outer londonderry' is the wordplay.
'a port authority' becomes 'pla' (Port of London Authority).
'outer' means to remove the middle letters.
'londonderry' with its middle removed is 'ly'.
'pla'+'in'+'ly'='PLAINLY'
'there's' is the link.
(Other definitions for plainly that I've seen before include "In unconcealed fashion" , "Clearly" , "Simply" , "obviously" , "... without embellishment" .)