Being strong requires firm heart in horrible surroundings (6)
I believe the answer is:
virile
'being strong' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I don't see?
'firm heart in horrible surroundings' is the wordplay.
'heart' says to take the centre.
'in horrible' becomes 'vile' (I've seen this in another clue. I am not sure about the 'in' bit.).
'surroundings' indicates putting letters inside.
The central letters of 'firm' are 'ir'.
'ir' inserted into 'vile' is 'VIRILE'.
'requires' is the link.
Can you help me to learn more?
(Other definitions for virile that I've seen before include "male" , "Robustly masculine" , "Manly, strong" , "Potent" , "Vigorous, manly" .)