By day, small boy gives shudder (7)
I believe the answer is:
frisson
'shudder' is the definition.
(I know that shudder can be written as frisson)
'by day small boy' is the wordplay.
'by' says to put letters next to each other.
'day' becomes 'fri' (abbreviation for Friday).
'small' becomes 's' (abbreviation - e.g. clothes size).
'boy' becomes 'son' (male child).
'fri'+'s' is 'fris'.
'fris'+'son'='FRISSON'
'gives' is the link.
(Other definitions for frisson that I've seen before include "Kick" , "A shudder or thrill" , "Shudder of excitement" , "Shiver of excitement" , "Strong feeling of excitement, thrill" .)