Irritate any number inside (5)
I believe the answer is:
annoy
'irritate' is the definition.
(I know that irritate can be written as annoy)
'any number inside' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'inside' means one lot of letters goes inside another.
'any' placed around 'no' is 'ANNOY'.
(Other definitions for annoy that I've seen before include "get angry" , "Make slightly angry" , "Irritate, needle" , "be a nuisance" , "Bother, irritate" .)