Only fish landlady gutted (6)
I believe the answer is:
solely
'only' is the definition.
(I know that only can be written as solely)
'fish landlady gutted' is the wordplay.
'fish' becomes 'sole' (sole is a kind of fish).
'gutted' suggests removing the centre (to gut something is to remove its insides).
'landlady' with its middle taken out is 'ly'.
'sole'+'ly'='SOLELY'
(Other definitions for solely that I've seen before include "Singly" , "nowhere else?" , "Singularly" , "Exclusively, only" , "alone" .)