Pressure mounting on firm given millions for making stuff up (10)

'making stuff up' is the definition.
(parts make up or comprise a whole)

'pressure mounting on firm given millions' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'mounting' becomes 'rising' ('rise' can be a synonym of 'mount').
'on' means one lot of letters go next to another.
'firm given millions' becomes 'com' (I can't justify this - if you can you should believe this answer much more).
'p'+'rising'='prising'
'prising' put after 'com' is 'COMPRISING'.

'for' is the link.