On one's guard, as accountant from Utah promises to pay for empty place (10)
I believe the answer is:
cautiously
'on one's guard as accountant from utah' is the definition.
I can't judge whether this definition defines the answer.
'promises to pay for empty place' is the wordplay.
'promises to pay' becomes 'cautious' (I am not sure about this - if you are sure you should believe this answer much more).
'for' means one lot of letters go next to another.
'empty' means to remove the middle letters.
'place' becomes 'lay' (laying is a kind of placing).
'lay' with its centre taken out is 'ly'.
'cautious'+'ly'='CAUTIOUSLY'
Can you help me to learn more?
(Another definition for cautiously that I've seen is " Warily".)