No firm's infirm, in brief (6)
I believe the answer is:
inform
'brief' is the definition.
(I know that brief is a more specific form of the action apprise)
'no firm's infirm' is the wordplay.
'infirm' indicates an anagram.
'no'+'firm'='nofirm'
'nofirm' is an anagram of 'INFORM'.
'in' is the link.
(Other definitions for inform that I've seen before include "Give information to (someone)" , "Pass on knowledge" , "Brief" , "Spill the beans" , "Let people know" .)