Linchpin might hold as tin disintegrates (8)
I believe the answer is:
mainstay
'linchpin' is the definition.
(I know that linchpin can be written as mainstay)
'might hold as tin disintegrates' is the wordplay.
'might' becomes 'may' (I've seen this before).
'hold' means one lot of letters goes inside another.
'disintegrates' indicates anagramming the letters (I've seen 'disintegrated' mean this).
'as'+'tin'='astin'
'astin' is an anagram of 'ainst'.
'may' placed around 'ainst' is 'MAINSTAY'.
(Other definitions for mainstay that I've seen before include "Chief support, like mast cable" , "Piece of rigging" , "Strong cable to secure mast, chief support" , "Strong cable that secures a mast, chief support generally" , "Strongest support" .)