Foolishly married in haste — regularly rues eagerness (10)
I believe the answer is:
enthusiasm
'eagerness' is the definition.
(thesaurus)
'foolishly married in haste regularly rues' is the wordplay.
'foolishly' is an anagram indicator.
'married' becomes 'm'.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'rues' are 'us'.
'm'+'in'+'haste'+'us'='minhasteus'
'minhasteus' with letters rearranged gives 'ENTHUSIASM'.
(Other definitions for enthusiasm that I've seen before include "Gusto; fervour" , "Zeal" , "Intense and lively interest" , "Ardour" , "Avidity" .)