Easy to cry when upset: it’s illuminating (5,4)
I believe the answer is:
light bulb
'it's illuminating' is the definition.
I can't tell whether this defines the answer.
'easy to cry when upset' is the wordplay.
'easy' becomes 'light' (similar in meaning).
'to cry' becomes 'blub' (I've seen this before).
'when upset' is a reversal indicator (in a down clue - set, written, up).
'blub' in reverse letter order is 'bulb'.
'light'+'bulb'='LIGHT-BULB'
Can you help me to learn more?
(Other definitions for light bulb that I've seen before include "One should make things clear" , "that'll brighten your surroundings" , "glower" , "may be subject to changing" , "(Electric) lamp" .)