Litmus perhaps in acid turned to red at first (9)
I believe the answer is:
indicator
'litmus perhaps' is the definition.
(litmus is a kind of indicator)
'in acid turned to red at first' is the wordplay.
'turned' shows that the letters should be reversed in order.
'at first' indicates taking the first letters.
The first letter of 'red' is 'r'.
'acid' reversed gives 'dica'.
'in'+'dica'+'to'+'r'='INDICATOR'
(Other definitions for indicator that I've seen before include "Station information board" , "sign" , "shower" , "This shows" , "Car's flashing light" .)