One small seabird beside cold reservoir (7)
I believe the answer is:
cistern
'reservoir' is the definition.
(I know that cistern is a type of reservoir)
'one small seabird beside cold' is the wordplay.
'one' becomes 'i' (Roman numeral).
'small' becomes 's' (abbreviation - e.g. clothes size).
'seabird' becomes 'tern' (tern is a kind of seabird).
'beside' means one lot of letters go next to another (I've seen this in other clues).
'cold' becomes 'c' (eg on taps).
'i'+'s'+'tern'='istern'
'istern' put after 'c' is 'CISTERN'.
(Other definitions for cistern that I've seen before include "Tank, reservoir" , "Toilet water reservoir" , "bit of toilet" , "WC water tank" , "Water tank for toilet" .)