Sex talk almost crucial (5)
I believe the answer is:
vital
'crucial' is the definition.
(I know that crucial can be written as vital)
'sex talk almost' is the wordplay.
'sex' becomes 'VI' (VI in Roman numerals - 'sex' is 'six' in Latin).
'almost' means to remove the last letter (almost the whole word is kept).
'talk' with its last letter removed is 'tal'.
'vi'+'tal'='VITAL'
(Other definitions for vital that I've seen before include "Spirited" , "Characteristic of life" , "Highly important" , "Crucial" , "Energetic - displaying life" .)