Short brat struggling to keep up (6)
I believe the answer is:
abrupt
'short' is the definition.
(abrupt can mean short or terse)
'brat struggling to keep up' is the wordplay.
'struggling' is an anagram indicator (the letters struggle or wriggle into a new form).
'to keep' is an insertion indicator.
'brat' with letters rearranged gives 'abrt'.
'abrt' placed around 'up' is 'ABRUPT'.
(Other definitions for abrupt that I've seen before include "Curt, sharp" , "Impolite" , "Unexpected - terse" , "Hurried or curt" , "Terse, curt" .)