Small flower captures heart of ample partner (6)
I believe the answer is:
spouse
'partner' is the definition.
('spouse' can be a synonym of 'partner')
'small flower captures heart of ample' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'flower' becomes 'ouse' (Yorkshire river**).
'captures' is an insertion indicator.
'heart of' means to look at the middle letters.
The centre of 'ample' is 'p'.
's'+'ouse'='souse'
'souse' enclosing 'p' is 'SPOUSE'.
(Other definitions for spouse that I've seen before include "Elizabethan dance" , "Partner in marriage" , "Possibly wife" , "mate" , "Other half" .)