Inventor showing regret when putting off engineers (5)
I believe the answer is:
morse
'inventor' is the definition.
(Samuel Morse, inventor of Morse code)
'regret when putting off engineers' is the wordplay.
'regret' becomes 'remorse' (remorse is a kind of regret).
'when putting off' suggests deleting specific letters.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'remorse' with 're' taken out is 'MORSE'.
'showing' is the link.
(Other definitions for morse that I've seen before include "Code using dots and dashes" , "Fictional inspector; code" , "Walrus - TV detective" , "Samuel ---, telegraph inventor" , "Radio code inventor" .)