Purple's second and blue's third in red fish (5)
I believe the answer is:
trout
'fish' is the definition.
(I know that trout is a type of fish)
'purple's second and blue's third in red' is the wordplay.
I cannot really understand how this works, but
'red' could be 'r' (I've seen this in other clues) and 'r' is located in the answer.
'blue's third' could be 'u' (3rd letter of 'blue') and 'u' is present in the answer.
The remaining letters 'tot' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for trout that I've seen before include "Tickled fish?" , "Oily fish" , "Freshwater game fish" , "Schubert Quintet?" , "tickled to death?" .)