Omit old Tory leader concealing fungal growth (6)
I believe the answer is:
except
'omit' is the definition.
(I know that omit can be written as except)
'old tory leader concealing fungal growth' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'leader' says to take the initial letters.
'concealing' indicates putting letters inside.
'fungal growth' becomes 'cep'.
The initial letter of 'tory' is 't'.
'ex'+'t'='ext'
'ext' going around 'cep' is 'EXCEPT'.
(Other definitions for except that I've seen before include "Exclude, take out" , "Save for" , "Other than - saving" , "Leaving out" , "Leave out, exclude" .)