Straight after rejection, tense setter's volcanic (8)
I believe the answer is:
eruptive
'volcanic' is the definition.
(synonyms)
'straight after rejection tense setter's' is the wordplay.
'straight after rejection tense' becomes 'erupt' (I am not sure about this - if you are sure you should believe this answer much more).
'setter's' becomes 'ive' ('I've', referring to the crossword setter).
'erupt'+'ive'='ERUPTIVE'
Can you help me to learn more?
(Other definitions for eruptive that I've seen before include "spouting" , "Like volcanoes" , "Like a gathering" , "liable to break out" .)