Fit girl running round bottom of garden, minor (8)
I believe the answer is:
trifling
'minor' is the definition.
(I've seen this before)
'fit girl running round bottom of garden' is the wordplay.
'running' indicates anagramming the letters.
'round' means one lot of letters goes inside another.
'bottom of' says to take the final letters.
The final letter of 'garden' is 'n'.
'fit'+'girl'='fitgirl'
'fitgirl' anagrammed gives 'triflig'.
'triflig' enclosing 'n' is 'TRIFLING'.
(Other definitions for trifling that I've seen before include "Inconsequential" , "Frivolous" , "Dallying; petty" , "Unimportant, trivial" , "Fooling around; unimportant" .)