Firm losing small data chart (5)
I believe the answer is:
table
'data chart' is the definition.
'firm losing small' is the wordplay.
'firm' becomes 'stable' ('stable' can be similar in meaning to 'firm').
'losing' is a deletion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'stable' with 's' removed is 'TABLE'.
(Other definitions for table that I've seen before include "Food put on this" , "Board off which dinner's eaten" , "Array of figures" , "League --; multiplication --" , "Flat surfaced piece of furniture" .)