Shook it at the old guard (8)
I believe the answer is:
agitated
'shook' is the definition.
('agitate' can be a synonym of 'shake')
'it at the old guard' is the wordplay.
I cannot quite understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
'at' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for agitated that I've seen before include "Aroused - disturbed" , "Upset, disturbed" , "Troubled, perturbed" , "Worried" , "caused a stir" .)