No.1 mounts, kept by trained equestrian in form (13)
I believe the answer is:
questionnaire
'form' is the definition.
(questionnaire is a kind of form)
'no 1 mounts kept by trained equestrian' is the wordplay.
'1' becomes 'i' (roman numeral).
'mounts' is a reversal indicator (I've seen 'mounting' mean this (going up in a down clue)).
'kept by' means one lot of letters goes inside another.
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'no'+'i'='noi'
'noi' back-to-front is 'ion'.
'equestrian' is an anagram of 'questnaire'.
'ion' put inside 'questnaire' is 'QUESTIONNAIRE'.
'in' acts as a link.
(Other definitions for questionnaire that I've seen before include "Survey fill-in form" , "Survey answer sheet" , "Form for collecting data" , "List of enquiries" , "List of inquiries" .)