Tense intensity containing end of nuclear menace (6)
I believe the answer is:
threat
'menace' is the definition.
('threat' can be a synonym of 'menace')
'tense intensity containing end of nuclear' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'intensity' becomes 'heat'.
'containing' is an insertion indicator.
'end of' suggests the final letters.
The last letter of 'nuclear' is 'r'.
'heat' placed around 'r' is 'hreat'.
't'+'hreat'='THREAT'
(Other definitions for threat that I've seen before include "Statement of intention to harm" , "The rat became a menace" , "Warning of intent to harm" , "Potential source of danger" , "Menacing words" .)