Ten cold in sea, about average (8)
I believe the answer is:
mediocre
'average' is the definition.
(I know that average can be written as mediocre)
'ten cold in sea about' is the wordplay.
'ten' becomes 'IO' (IO resembles r0).
'cold' becomes 'c' (eg on taps).
'in' indicates putting letters inside.
'sea' becomes 'med' (short for Mediterranean).
'about' becomes 're' ('re' means regarding or about).
'io'+'c'='ioc'
'med'+'re'='medre'
'ioc' placed into 'medre' is 'MEDIOCRE'.
(Other definitions for mediocre that I've seen before include "Somewhat inferior" , "poor to middling" , "Of middling quality" , "Fair, middle of the road" , "Commonplace, not distinguished" .)