Animals second rate? (6)
I believe the answer is:
assess
'rate?' is the definition.
(rating is a kind of assessing)
'animals second' is the wordplay.
'animals' becomes 'asses' (I've seen this before).
'second' becomes 's' (common abbreviation).
'asses'+'s'='ASSESS'
(Other definitions for assess that I've seen before include "Estimate the worth or quality of" , "Reckon value" , "Rate, evaluate" , "Test" , "Judge the quality of" .)