Aged fellow starts to collapse (4)
I believe the answer is:
fold
'to collapse' is the definition.
(as in a business folding)
'aged fellow starts' is the wordplay.
'aged' becomes 'old' (similar in meaning).
'fellow' becomes 'F' (abbreviation - of a society etc.).
'starts' means one lot of letters go next to another.
'old' after 'f' is 'FOLD'.
(Other definitions for fold that I've seen before include "stop functioning" , "Church following" , "Congregation; (firm) go bankrupt" , "Members of church" , "Collapse; bend in half" .)