Simple shirt, a “large”, to keep young child dry (8)
I believe the answer is:
teetotal
'dry' is the definition.
(I know that dry can be written as teetotal)
'simple shirt a large to keep young child' is the wordplay.
'simple shirt' becomes 'tee' (I've seen this in another clue).
'large' becomes 'L' (eg in clothes sizes).
'to keep' means one lot of letters goes inside another.
'young child' becomes 'tot' (tot can mean a baby or toddler).
'tee'+'a'+'l'='teeal'
'teeal' going around 'tot' is 'TEETOTAL'.
(Other definitions for teetotal that I've seen before include "Sober" , "Abstemious" , "Eschewing booze" , "On the wagon" , "Abjuring alcohol" .)