Figure poor-quality stuff is involved in a lot of criticism (9)
I believe the answer is:
statistic
'lot of criticism' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'figure poor-quality stuff is involved' is the wordplay.
'figure' becomes 'stat' (statistic).
'poor quality stuff' becomes 'tic' (I can't explain this - if you can you should believe this answer much more).
'involved' means one lot of letters goes inside another.
'stat'+'tic'='stattic'
'stattic' enclosing 'is' is 'STATISTIC'.
'in a' is the link.
(Other definitions for statistic that I've seen before include "Data point" , "E.g. crime figure" , "It could be vital" , "Numerical fact" , "Collector's item" .)