Promising a fishy with head removed (10)
I believe the answer is:
auspicious
'promising' is the definition.
(similar in meaning)
'a fishy with head removed' is the wordplay.
'fishy' becomes 'suspicious' (synonyms).
'with head removed' means to remove the first letter.
'suspicious' with its first letter taken away is 'uspicious'.
'a'+'uspicious'='AUSPICIOUS'
(Other definitions for auspicious that I've seen before include "Promising" , "Rosy" , "Favourable" .)