Pores over a child’s first bits of artwork, messily clutched (7)
I believe the answer is:
stomata
'pores' is the definition.
('stoma' can be a synonym of 'pore')
'over a child's first bits of artwork messily clutched' is the wordplay.
'over' is a reversal indicator.
'child's' becomes 'tots' (term for a young child).
'first bits of' indicates taking the first letters (I've seen 'first bit' mean this).
'clutched' indicates putting letters inside.
The initial letters of 'artwork messily' is 'am'.
'a'+'tots'='atots'
'atots' going around 'am' is 'atamots'.
'atamots' reversed gives 'STOMATA'.
(Other definitions for stomata that I've seen before include "Openings" , "Plant pores" , "Minute leaf pore" .)