Brief Conservative following terrible strain (10)
I believe the answer is:
transitory
'brief' is the definition.
(I've seen this before)
'conservative following terrible strain' is the wordplay.
'conservative' becomes 'tory' (I've seen this before).
'following' means one lot of letters go next to another.
'terrible' indicates anagramming the letters.
'strain' is an anagram of 'transi'.
'tory' after 'transi' is 'TRANSITORY'.
(Other definitions for transitory that I've seen before include "Brief" , "Passing" , "Short-lived" , "Fleeting" , "Here-today-gone-tomorrow" .)