Prosecute light infantry pulled up for obstruction (5)
I believe the answer is:
ileus
'obstruction' is the definition.
(I know that ileus is a type of obstruction)
'prosecute light infantry pulled up' is the wordplay.
'prosecute' becomes 'sue' (I've seen this in another clue).
'light infantry' becomes 'li' (abbreviation).
'pulled up' says the letters should be written in reverse.
'sue'+'li'='sueli'
'sueli' back-to-front is 'ILEUS'.
'for' acts as a link.
(Other definitions for ileus that I've seen before include "A bad gut feeling" , "Medical condition" , "A nasty gut feeling" .)