Hang on desperately in case of lad writing (8)
I believe the answer is:
longhand
'writing' is the definition.
(longhand is a kind of writing)
'hang on desperately in case of lad' is the wordplay.
'desperately' is an anagram indicator.
'in' indicates putting letters inside.
'case of' means to remove the middle letters (outsides of).
'lad' with its middle taken out is 'ld'.
'hang'+'on'='hangon'
'hangon' with letters rearranged gives 'onghan'.
'onghan' going inside 'ld' is 'LONGHAND'.
(Other definitions for longhand that I've seen before include "Ordinary writing - not typing or printing" , "Having words written out in full by hand" , "Ordinary writing of manual origin" , "script" , "Joined-up writing" .)