Wholly alert, he got cracking (10)
I believe the answer is:
altogether
'wholly' is the definition.
('altogether' can be a synonym of 'wholly')
'alert he got cracking' is the wordplay.
'cracking' is an anagram indicator.
'alert'+'he'+'got'='alerthegot'
'alerthegot' anagrammed gives 'ALTOGETHER'.
(Other definitions for altogether that I've seen before include "Completely, in total" , "Wholly" , "nude in this" , "Entirely" , "totally" .)