Twenty-four hours without any answers for firm (6)
I believe the answer is:
sturdy
'for firm' is the definition.
(firm can mean sturdy or solid)
I can't explain the remainder of the clue.
(Other definitions for sturdy that I've seen before include "Hardy, stalwart" , "Powerful" , "Strongly and solidly built" , "Well-constructed" , "Firmly constructed" .)