Yard excavation turns frivolous (5)
I believe the answer is:
giddy
'frivolous' is the definition.
('giddy' can be similar in meaning to 'frivolous')
'yard excavation turns' is the wordplay.
'yard' becomes 'yd'.
'excavation' becomes 'dig' ('dig' can be a synonym of 'excavation').
'turns' shows that the letters should be reversed in order.
'yd'+'dig'='yddig'
'yddig' reversed gives 'GIDDY'.
(Other definitions for giddy that I've seen before include "Flighty or lightheaded." , "Suffering from vertigo" , "Sturdy" , "Frivolous -- liable to fall" , "With spinning head" .)