Significant life gunman destroyed (10)
I believe the answer is:
meaningful
'significant' is the definition.
('meaningful' can be similar in meaning to 'significant')
'life gunman destroyed' is the wordplay.
'destroyed' is an anagram indicator.
'life'+'gunman'='lifegunman'
'lifegunman' is an anagram of 'MEANINGFUL'.
(Other definitions for meaningful that I've seen before include "Important" , "Significant" .)