Clumsy and sluggish tackling ten exercise classes (8)
I believe the answer is:
inexpert
'clumsy' is the definition.
(I've seen this before)
'sluggish tackling ten exercise classes' is the wordplay.
'sluggish' becomes 'inert' (synonyms).
'tackling' is an insertion indicator (to tackle is to take possession of).
'ten' becomes 'X' (Roman numeral).
'exercise classes' becomes 'PE' (Physical Education).
'x'+'pe'='xpe'
'inert' going around 'xpe' is 'INEXPERT'.
'and' acts as a link.
(Other definitions for inexpert that I've seen before include "non-professional" , "Green" , "Unskilled" , "with little skill" , "Not skilled or well-versed" .)