Engineers quiet about his heart being strong (9)
I believe the answer is:
resilient
'strong' is the definition.
(resilient can mean strong or tough)
'engineers quiet about his heart' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'quiet' becomes 'silent' (similar in meaning).
'about' indicates putting letters inside.
'heart' indicates the central letters.
The middle letter of 'his' is 'i'.
'silent' placed around 'i' is 'silient'.
're'+'silient'='RESILIENT'
'being' acts as a link.
(Other definitions for resilient that I've seen before include "Rebounding from impact" , "able to withstand disappointment" , "Bouncy" , "Adaptable" , "Able to recover quickly" .)