Fatal delay, perhaps, involves a number (6)
I believe the answer is:
deadly
'fatal' is the definition.
(similar in meaning)
'delay perhaps involves a number' is the wordplay.
'perhaps' indicates anagramming the letters.
'involves' is an insertion indicator.
'a number' becomes 'd' (D is an example).
'delay' with letters rearranged gives 'dealy'.
'dealy' enclosing 'd' is 'DEADLY'.
(Other definitions for deadly that I've seen before include "Lethal, could be fatal" , "Belladonna" , "Lethal, capable of killing" , "sniper aspires to be" , "Lethal or mortal" .)