10 + 10 divided by small number? That's a strain (7)
I believe the answer is:
tension
'a strain' is the definition.
(tension is a kind of strain)
'10 10 divided by small number?' is the wordplay.
'10' becomes 'ten' ('ten' can be a synonym of '10').
'10' becomes 'io' (I've seen this in other clues).
'divided by' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'number?' becomes 'n'.
'ten'+'io'='tenio'
'tenio' placed around 's' is 'tensio'.
'tensio'+'n'='TENSION'
'that's' acts as a link.
(Other definitions for tension that I've seen before include "Anxiety" , "One isn't (anag.)" , "Assay" , "Uneasiness" , "Stretched, uneasy, condition" .)