Mock exam that is opened by retired teacher (8)
I believe the answer is:
satirise
'mock' is the definition.
('mock'->'satirize' is in my internal thesaurus and 'ize'->'ise')
'exam that is opened by retired teacher' is the wordplay.
'exam' becomes 'SAT' (standard attainment test, UK exam).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'opened by' means one lot of letters goes inside another (some letters open a gap in others).
'retired' says the letters should be written backwards.
'teacher' becomes 'sir' (a male teacher might be addressed as 'sir').
'sir' back-to-front is 'ris'.
'ie' enclosing 'ris' is 'irise'.
'sat'+'irise'='SATIRISE'
(Other definitions for satirise that I've seen before include "parody" , "Ridicule with witty words" , "Criticise with a literary composition" , "take off" , "Ridicule with irony" .)