Crossword Genius

Mustn’t hope for me to change ad hoc (4,2,3,6)

Ross

I believe the answer is:

spur of the moment

I'm a little stuck... Click here to teach me more about this clue!

'ad hoc' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I don't see?

'mustn't hope for me to change' is the wordplay.
'to change' is an anagram indicator.
'mustnt'+'hope'+'for'+'me'='mustnthopeforme'
'mustnthopeforme' with letters rearranged gives 'SPUR-OF-THE-MOMENT'.

Can you help me to learn more?

(Other definitions for spur of the moment that I've seen before include "Unplanned" , "Impulsive" , "in-form player at White Hart Lane?" , "Possibly Gareth Bale" , "impulse?" .)

I've seen this clue in the Sydney Morning Herald.
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