When in the collapsing shed, makes a run for it (6)
I believe the answer is:
dashes
'makes a run for it' is the definition.
(I've seen this in another clue)
'when in the collapsing shed' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'in' means one lot of letters goes inside another.
'the collapsing' indicates anagramming the letters.
'shed' is an anagram of 'dhes'.
'as' put inside 'dhes' is 'DASHES'.
(Other definitions for dashes that I've seen before include "Quick run or sudden rush" , "hares" , "Long signals in Morse code" , "flies!" , "Strokes" .)