The incongruity of a very strong end to a story (5)
I believe the answer is:
irony
'the incongruity' is the definition.
(irony is a kind of incongruity)
'a very strong end to a story' is the wordplay.
'a very strong' becomes 'iron' (I've seen this before).
'end to' indicates one should take the final letters.
The final letter of 'story' is 'y'.
'iron'+'y'='IRONY'
'of' acts as a link.
(Other definitions for irony that I've seen before include "Satire, sarcasm" , "Scornful wit" , "Kind of sarcasm, sounds metallic" , "Mild sarcasm" , "Dry wit" .)