Dormant fish turned into a snake (6)
I believe the answer is:
asleep
'dormant' is the definition.
(similar in meaning)
'fish turned into a snake' is the wordplay.
'fish' becomes 'eel' (eel is a kind of fish).
'turned' shows that the letters should be reversed in order.
'into' indicates putting letters inside.
'a snake' becomes 'asp' (asp is a kind of snake).
'eel' in reverse letter order is 'lee'.
'lee' put within 'asp' is 'ASLEEP'.
(Other definitions for asleep that I've seen before include "Like kippers" , "In the Land of Nod" , "coiled up?" , "not paying attention" , "Dormant, inactive" .)