Go to collect new guides — they're probably in the office (10)
I believe the answer is:
penpushers
'they're probably in the office' is the definition.
(a penpusher is someone with an office job)
'go to collect new guides' is the wordplay.
I cannot really see how this works, but
'new' could be 'n' (common abbreviation eg NT for New Testament) and 'n' is found in the answer.
'go' could be 'pep' (I've seen this in another clue) and 'pep' is located in the leftover letters.
'guides' could be 'ushers' (ushering is a kind of guiding) and 'ushers' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
(Another definition for penpushers that I've seen is " Clerks who do boring paperwork".)