Ladders comprising ultimately strong steps (5)
I believe the answer is:
rungs
'steps' is the definition.
(I've seen this before)
'ladders comprising ultimately strong' is the wordplay.
'ladders' becomes 'runs' ('run' can be a synonym of 'ladder').
'comprising' indicates putting letters inside.
'ultimately' suggests the final letters.
The final letter of 'strong' is 'g'.
'runs' placed around 'g' is 'RUNGS'.
(Other definitions for rungs that I've seen before include "Ladder parts" , "Crosspieces of a ladder" , "Steps of a ladder" , "bars" .)