Sturdy press employee stumbling in at last (11)
I believe the answer is:
substantial
'sturdy' is the definition.
('substantial' can be a synonym of 'sturdy')
'press employee stumbling in at last' is the wordplay.
'press employee' becomes 'sub' (subeditor).
'stumbling' indicates anagramming the letters.
'in'+'at'+'last'='inatlast'
'inatlast' with letters rearranged gives 'stantial'.
'sub'+'stantial'='SUBSTANTIAL'
(Other definitions for substantial that I've seen before include "Of considerable importance" , "Of considerable size" , "Solid" .)