Feels sick at the opening and leaves, gutted, in order to be considerate (8)
I believe the answer is:
selfless
'be considerate' is the definition.
(I've seen this before)
'feels sick at the opening and leaves gutted in order' is the wordplay.
'at the opening' suggests taking the first letters.
'and' says to put letters next to each other.
'gutted' means to remove the middle letters (to gut something is to remove its insides).
'in order' is an anagram indicator (the letters in a new order).
'leaves' with its middle taken out is 'ls'.
The initial letter of 'sick' is 's'.
'feels'+'s'+'ls'='feelssls'
'feelssls' with letters rearranged gives 'SELFLESS'.
'to' acts as a link.
(Other definitions for selfless that I've seen before include "Generous" , "Altruistic" , "Considerate" .)