Existed on pastry and radishes, started to become more petulant (7)

'more petulant' is the definition.
Both the definition and answer are comparative adjectives.
Perhaps there's an association between them I don't understand?

'existed on pastry and radishes started' is the wordplay.
'existed' becomes 'was' ('be' can be a synonym of 'exist').
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'pastry' becomes 'pie' (pie is a kind of pastry).
'and' says to put letters next to each other.
'started' suggests taking the first letters.
The first letter of 'radishes' is 'r'.
'was'+'pie'+'r'='WASPIER'

'to become' is the link.

Can you help me to learn more?