This insect makes 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' (I've seen this before**).
'up' indicates an anagram.
'sit' is an anagram of 'tis'.
'man'+'tis'='MANTIS'
'makes' is the link.
(Other definitions for mantis that I've seen before include "Insect in attitude of prayer" , "Large insect (sometimes praying?)" , "Actually it preys rather than prays" , "may be praying" , "It appears to be praying but is more likely preying" .)