Engineers condescend to accept second new plan (8)
I believe the answer is:
redesign
'new plan' is the definition.
(I've seen this before)
'engineers condescend to accept second' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'condescend' becomes 'deign' (synonyms).
'to accept' means one lot of letters goes inside another.
'second' becomes 's' (common abbreviation).
'deign' placed around 's' is 'design'.
're'+'design'='REDESIGN'
(Other definitions for redesign that I've seen before include "Plan something again" , "Plan again differently" , "Make new plans" , "new plan" , "Modernise" .)