Some catch up (4)
I believe the answer is:
part
'some' is the definition.
(thesaurus)
'catch up' is the wordplay.
'catch' becomes 'trap' (trapping is a kind of catching).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'trap' in reverse letter order is 'PART'.
(Other definitions for part that I've seen before include "Divide; role" , "Constituent - something to play?" , "unemployed actor hopes for" , "Fraction of the whole - separate" , "Region" .)