Big and bright benevolent heart at first (10)
I believe the answer is:
voluminous
'big' is the definition.
(similar in meaning)
'bright benevolent heart at first' is the wordplay.
'bright' becomes 'luminous' (similar in meaning).
'heart' indicates the central letters.
'at first' says to put letters next to each other.
The centre of 'benevolent' is 'vo'.
'luminous' after 'vo' is 'VOLUMINOUS'.
'and' acts as a link.
(Other definitions for voluminous that I've seen before include "roomy" , "Filling a lot of space" .)