Might it be full of Avon water? (4)
I believe the answer is:
bath
'water?' is the definition.
(bath water)
'might it be full of avon' is the wordplay.
I cannot quite see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
The remaining letters 'bah' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for bath that I've seen before include "City of SW England" , "Home fixture (for City?)" , "Tub for washing" , "English city - tub" , "city that may or may not chime with 27 [HEARTH]" .)