Exacting short-term drills start off aerobic exercise (7)
I believe the answer is:
austere
'exacting' is the definition.
(thesaurus)
'short-term drills start off aerobic exercise' is the wordplay.
'short' means to remove the last letter.
'drills' is an insertion indicator (to drill can mean to make a hole in something).
'start off' says to take the initial letters.
'exercise' becomes 'use' (synonyms).
The initial letter of 'aerobic' is 'a'.
'term' with its last letter removed is 'ter'.
'a'+'use'='ause'
'ter' placed within 'ause' is 'AUSTERE'.
(Other definitions for austere that I've seen before include "Abstemious and grave" , "Severe, ascetic" , "Stern or very plain" , "not of luxury type" , "severely self-disciplined" .)