Useful old exercise gets Ted in trouble (9)
I believe the answer is:
expedient
'useful' is the definition.
(associated in meaning)
'old exercise gets ted in trouble' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'exercise' becomes 'pe' (Physical Education).
'gets' means one lot of letters goes inside another.
'trouble' indicates an anagram.
'ted'+'in'='tedin'
'tedin' is an anagram of 'dient'.
'pe' put within 'dient' is 'pedient'.
'ex'+'pedient'='EXPEDIENT'
(Other definitions for expedient that I've seen before include "something for emergency use" , "Means to get" , "Practical - advantageous" , "Suited to the circumstances" , "Convenient rather than right" .)