Make new addition to old-fashioned suit (8)
I believe the answer is:
retrofit
'make new addition to' is the definition.
I can't tell whether this defines the answer.
'old-fashioned suit' is the wordplay.
'old-fashioned' becomes 'retro' (retro can mean reminiscent of past fashions).
'suit' becomes 'fit' (synonyms).
'retro'+'fit'='RETROFIT'
Can you help me to learn more?
(Other definitions for retrofit that I've seen before include "Add (component) not originally in place" , "Fit with accessory not on original" , "Add (a component) not in the original build" , "Make later modifications to (equipment, etc)" , "Belatedly modify" .)