One adult held by military engineers after old penny vanishes (10)
I believe the answer is:
disappears
'vanishes' is the definition.
('disappear' can be a synonym of 'vanish')
'one adult held by military engineers after old penny' is the wordplay.
'one adult' becomes 'a' (abbreviation).
'held by' is an insertion indicator.
'military engineers' becomes 'sappers' (a sapper is a soldier who works as an engineer).
'after' says to put letters next to each other.
'old penny' becomes 'di' (I can't explain this - if you can you should believe this answer much more).
'a' going into 'sappers' is 'sappears'.
'sappears' after 'di' is 'DISAPPEARS'.
Can you help me to learn more?
(Other definitions for disappears that I've seen before include "becomes lost" , "goes missing" .)