One loading gun has developed distress (7)
I believe the answer is:
anguish
'distress' is the definition.
(I know that anguish is a type of distress)
'one loading gun has developed' is the wordplay.
'one' becomes 'i' (Roman numeral).
'loading' is an insertion indicator (some letters are loaded into others).
'developed' is an anagram indicator.
'gun'+'has'='gunhas'
'gunhas' is an anagram of 'angush'.
'i' put inside 'angush' is 'ANGUISH'.
(Other definitions for anguish that I've seen before include "extreme pain" , "Extreme misery" , "torture" , "Extreme torment" , "Hi, Angus - that hurts" .)