Groups gathering information — about a thousand pieces (8)
I believe the answer is:
segments
'pieces' is the definition.
(segment is a kind of piece)
'groups gathering information about a thousand' is the wordplay.
'groups' becomes 'sets' (I've seen this in other clues).
'gathering' means one lot of letters goes inside another.
'information' becomes 'gen' (gen can informally mean information).
'about' means one lot of letters goes inside another.
'a thousand' becomes 'M' (Roman numeral for a thousand).
'sets' going around 'gen' is 'segents'.
'segents' enclosing 'm' is 'SEGMENTS'.
(Other definitions for segments that I've seen before include "Parts, pieces, portions" , "'Sections, pieces (8)'" , "Divides or splits up" , "Sections, portions" , "Parts or pieces that fit with others" .)