Lots of information somewhat rejected by firm (4,3)
I believe the answer is:
data set
'lots of information' is the definition.
(data is information)
'somewhat rejected by firm' is the wordplay.
'somewhat' becomes 'atad' ('a tad' can mean 'somewhat' or 'slightly').
'rejected' is a reversal indicator.
'by' means one lot of letters go next to another.
'firm' becomes 'set' (both can mean fixed).
'atad' in reverse letter order is 'data'.
'data'+'set'='DATA SET'
(Other definitions for data set that I've seen before include "Collection of information (in computing)" , "file" , "collected info" .)