Firm dropping case of security guards (6)
I believe the answer is:
sturdy
'firm' is the definition.
(I've seen this before)
'dropping case of security guards' is the wordplay.
'dropping' becomes 'turd' (**).
'case of' says to hollow out the word (remove centre letters) (outsides of).
'guards' means one lot of letters goes inside another.
'security' with its centre taken out is 'sy'.
'turd' inserted inside 'sy' is 'STURDY'.
(Other definitions for sturdy that I've seen before include "Strong and reliable" , "Strong and stout" , "Hardy, stalwart" , "Rugged" , "Uncompromising" .)