Thus returning seabird in reservoir (7)
I believe the answer is:
cistern
'reservoir' is the definition.
(cistern is a kind of reservoir)
'thus returning seabird' is the wordplay.
'thus' becomes 'sic' ('thus' in Latin).
'returning' is a reversal indicator.
'seabird' becomes 'tern' (tern is a kind of seabird).
'sic' written backwards gives 'cis'.
'cis'+'tern'='CISTERN'
'in' is the link.
(Other definitions for cistern that I've seen before include "Toilet tank [ital]" , "Tank, reservoir" , "bit of toilet" , "Water storage tank" , "Tank - sac of body fluid" .)