Dormant fish turned into snake (6)
I believe the answer is:
asleep
'dormant' is the definition.
('asleep' can be similar in meaning to 'dormant')
'fish turned into snake' is the wordplay.
'fish' becomes 'eel' (eel is a kind of fish**).
'turned' shows that the letters should be reversed in order.
'into' means one lot of letters goes inside another.
'snake' becomes 'asp' (asp is a kind of snake).
'eel' written backwards gives 'lee'.
'lee' going into 'asp' is 'ASLEEP'.
(Other definitions for asleep that I've seen before include "Napping" , "Slumbering" , "In the Land of Nod" , "Dormant, inactive" , "Please be at rest (6)." .)