Worry a number offend, but there's nothing in it (7)
I believe the answer is:
tension
'worry' is the definition.
(both can mean stress or unease)
'a number offend but there's nothing in it' is the wordplay.
'a number' becomes 'ten' (ten is a number).
'offend but' becomes 'sin' (sin can mean to commit an offence. I am not sure about the 'but' bit.).
'there's nothing' becomes 'o' (I've seen this in other clues).
'in it' indicates putting letters inside.
'sin' enclosing 'o' is 'sion'.
'ten'+'sion'='TENSION'
Can you help me to learn more?
(Other definitions for tension that I've seen before include "Assay" , "Stretched, uneasy, condition" , "Stress, tautness" , "Strained condition" , "Stretched state" .)