Element of surprise, initially therein a memoir isn't true (9,6)

'element' is the definition.
(I know that immersion heater is a type of heating element)

'surprise initially therein a memoir isn't true' is the wordplay.
'initially' says to take the initial letters.
'isn't true' indicates an anagram (false form of the letters).
The first letter of 'surprise' is 's'.
's'+'therein'+'a'+'memoir'='sthereinamemoir'
'sthereinamemoir' with letters rearranged gives 'IMMERSION HEATER'.

'of' acts as a link.