Soldiers felt bad about source of crack in mirror (7)
I believe the answer is:
reflect
'mirror' is the definition.
(thesaurus)
'soldiers felt bad about source of crack' is the wordplay.
'soldiers' becomes 're' (Royal Engineers**).
'bad' is an anagram indicator.
'about' is an insertion indicator.
'source of' says to take the initial letters.
The first letter of 'crack' is 'c'.
'felt' with letters rearranged gives 'flet'.
'flet' enclosing 'c' is 'flect'.
're'+'flect'='REFLECT'
'in' acts as a link.
(Other definitions for reflect that I've seen before include "Think - send back" , "Ponder - mirror" , "Ponder; send back" , "Throw back (light, etc)" , "Meditate -- throw back" .)