A reliable firm (6)
I believe the answer is:
steady
This clue is a double definition.
'a reliable' is the first definition.
(I've seen this before)
'firm' is the second definition.
(similar in meaning)
(Other definitions for steady that I've seen before include "Unfluctuating" , "Stable and reliable" , "Unwavering" , "On an even keel" , "Maybe 'staid'" .)