Showing weakness for first-rate fish (6)
I believe the answer is:
ailing
'showing weakness' is the definition.
Although both the answer and definition are present participle verbs, I cannot understand how one could define the other.
'first-rate fish' is the wordplay.
'first-rate' becomes 'ai' (resembles A1, informal term for excellent).
'fish' becomes 'ling' (ling is a kind of fish).
'ai'+'ling'='AILING'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for ailing that I've seen before include "Ill" , "Troubled in mind or body" , "Infertile" , "not doing very well" , "In poor health" .)