Practical once confined to peg out inside (9)
I believe the answer is:
expedient
'practical' is the definition.
(thesaurus)
'once confined to peg out inside' is the wordplay.
'once' becomes 'ex' (prefix meaning former or onetime).
'confined' becomes 'pent' (similar in meaning).
'to peg out' becomes 'die'.
'inside' indicates putting letters inside.
'pent' placed around 'die' is 'pedient'.
'ex'+'pedient'='EXPEDIENT'
(Other definitions for expedient that I've seen before include "Advisable" , "Convenient, practical" , "Fitting" , "Appropriate, convenient" , "shift" .)