Dicky posits an idea about opening of summer fair (13)
I believe the answer is:
dispassionate
'fair' is the definition.
('dispassionate' can be similar in meaning to 'fair')
'dicky posits an idea about opening of summer' is the wordplay.
'dicky' indicates an anagram (dicky can mean ill or non-functioning).
'about' is an insertion indicator.
'opening of' indicates taking the first letters.
The first letter of 'summer' is 's'.
'posits'+'an'+'idea'='positsanidea'
'positsanidea' anagrammed gives 'dispasionate'.
'dispasionate' placed around 's' is 'DISPASSIONATE'.
(Other definitions for dispassionate that I've seen before include "Rational, impartial" , "taking the unbiased view?" , "Not affected by personal feelings" , "Imperturbable" , "Objective, unemotional" .)