Island seabird seen below Cape reservoir? (7)
I believe the answer is:
cistern
'reservoir?' is the definition.
(I know that cistern is a type of reservoir)
'island seabird seen below cape' is the wordplay.
'island' becomes 'is' (abbreviation).
'seabird' becomes 'tern' (tern is a kind of seabird).
'seen below' says to put letters next to each other (some letters go below others).
'cape' becomes 'c' (abbreviation used on maps).
'is'+'tern'='istern'
'istern' after 'c' is 'CISTERN'.
(Other definitions for cistern that I've seen before include "Tank - sac of body fluid" , "Toilet water reservoir" , "Water tank for toilet" , "Toilet tank [ital]" , "Water container" .)