Angler served up trouble for fish (6)
I believe the answer is:
dorado
'fish' is the definition.
(another name for the mahi-mahi fish)
'angler served up trouble' is the wordplay.
'angler' becomes 'rod' (I can't justify this - if you can you should believe this answer much more).
'served up' is a reversal indicator (in a down clue, going upwards).
'trouble' becomes 'ado' (both can mean bother or commotion).
'rod' backwards is 'dor'.
'dor'+'ado'='DORADO'
'for' is the link.
Can you help me to learn more?
(Other definitions for dorado that I've seen before include "'El ..., imaginary place of great wealth (6)'" , "stars" , "El ..., place of fabulous wealth or great opportunity" , "Fish; do a rod (anag.)" , "Marine fish" .)