Ladder lets one escape from dilapidated building (3)
I believe the answer is:
run
'ladder' is the definition.
(both can mean a tear in tights)
'one escape from dilapidated building' is the wordplay.
'one' becomes 'I' (Roman numeral).
'escape from' suggests deleting specific letters.
'dilapidated building' becomes 'ruin' (dilapidated or ruined building).
'ruin' with 'i' removed is 'RUN'.
'lets' acts as a link.
(Other definitions for run that I've seen before include "Wide" , "Move faster than walking" , "Flow quickly" , "Administer" , "Ladder - manage - sprint - flow" .)