Promoted one internally in firm (5)
I believe the answer is:
solid
'firm' is the definition.
('solid' can be a synonym of 'firm')
'promoted one internally' is the wordplay.
'promoted' becomes 'sod' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one' becomes 'li' (I've seen this in other clues).
'internally' indicates putting letters inside.
'sod' going around 'li' is 'SOLID'.
'in' is the link.
Can you help me to learn more?
(Other definitions for solid that I've seen before include "from one getting pushed around" , "Weighty" , "Not hollow, the same throughout" , "Reliable - strong" , "Idols (anag.)" .)