Locomotive to fail: no good stopping German one (4,6)
I believe the answer is:
tank engine
'locomotive' is the definition.
(I know that tank engine is a type of locomotive)
'fail no good stopping german one' is the wordplay.
'fail' becomes 'tank' (informal term meaning to fail financially).
'no good' becomes 'ng' (abbreviation).
'stopping' is an insertion indicator (stopping can mean blocking a gap).
'german one' becomes 'eine' (I've seen this in another clue).
'ng' placed within 'eine' is 'engine'.
'tank'+'engine'='TANK-ENGINE'
'to' is the link.
(Other definitions for tank engine that I've seen before include "Possibly Thomas" , "Thomas, say" , "army vehicle power unit" , "Thomas, perhaps" .)