Second truss? (7)
I believe the answer is:
support
This clue is a double definition.
'second' is the first definition.
(I've seen this before)
'truss?' is the second definition.
(trussing is a kind of supporting)
(Other definitions for support that I've seen before include "Keep from falling, as crutch does (7)" , "Give assistance to" , "Bolster or back up" , "Stay" , "Provide financial backing or bolster" .)