Summaries concerning upper limits (6)
I believe the answer is:
recaps
'summaries' is the definition.
(recap is a kind of summary)
'concerning upper limits' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'upper limits' becomes 'caps' (I've seen this in another clue).
're'+'caps'='RECAPS'
(Other definitions for recaps that I've seen before include "Such summaries" , "In brief, state again the main points" , "Summarises at the end" , "Reviews" , "Summarises briefly" .)