School has nothing certain to provide tranquillity (9)
I believe the answer is:
composure
'tranquillity' is the definition.
(I've seen this before)
'school has nothing certain' is the wordplay.
'school' becomes 'comp' (short for comprehensive school).
'has nothing' becomes 'o' (I've seen this in other clues).
'certain' becomes 'sure' (synonyms).
'comp'+'o'+'sure'='COMPOSURE'
'to provide' is the link.
(Other definitions for composure that I've seen before include "Calm state" , "tranquillity" , "Coolness" , "Calmness and self-control" , "Calm self-control" .)