Timeless part of flower as symbol of summer? (5)
I believe the answer is:
sigma
'symbol of summer?' is the definition.
(mathematical symbol used by people doing sums)
'timeless part of flower' is the wordplay.
I cannot really see how this works, but
'part' could be 'si' (si is a kind of part) and 'si' is located in the answer.
This explanation may well be incorrect...
'as' acts as a link.
(Other definitions for sigma that I've seen before include "Allowing no relaxation" , "Greek letter signifying sum in maths" , "Greek letter corresponding to S" , "Greek letter used as summation sign" , "character abroad" .)