This insect will make a chap sit up! (6)
I believe the answer is:
mantis
'this insect' is the definition.
(mantis is a kind of insect)
'a chap sit up' is the wordplay.
'a chap' becomes 'man' (chap can mean a man).
'up' indicates anagramming the letters (letters get chucked up).
'sit' is an anagram of 'tis'.
'man'+'tis'='MANTIS'
'will make' is the link.
(Other definitions for mantis that I've seen before include "Insect of the cockroach family" , "may be praying" , "Carnivorous insect, which rests as if in prayer" , "Predatory insect" , "Preying animal, the female of which is deadlier than the male!" .)