Seeing some burglar get nicked on the way up is essential (8)
I believe the answer is:
integral
'essential' is the definition.
(I've seen this before)
'seeing some burglar get nicked on the way up' is the wordplay.
'seeing some' says the answer is hidden in the clue (some of the parts of the word are used).
'on the way up' says the letters should be written in reverse.
'largetni' is hidden in the letters of 'burglar get nicked'.
'largetni' backwards is 'INTEGRAL'.
'is' is the link.
(Other definitions for integral that I've seen before include "using no fractions" , "Forming part of the whole" , "Of part necessary to the completeness of a whole" , "United" , "Formed of constituent parts" .)