Summer featured days on the turn, shorter (5)
I believe the answer is:
adder
'summer' is the definition.
(someone who adds or does sums)
'featured days on the turn shorter' is the wordplay.
'ADDER' is hidden back-to-front in amongst the letters.
But, I am not sure how this is indicated.
Can you help me to learn more?
(Other definitions for adder that I've seen before include "Venemous snake" , "Small poisonous snake, viper" , "He's making an extension" , "Something you have to reckon with" , "exhibit in the reptile house" .)