Attempt to introduce the second of horses here? (4)
I believe the answer is:
troy
'here?' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'attempt to introduce the second of horses' is the wordplay.
'attempt' becomes 'try' (synonyms).
'to introduce' is an insertion indicator (to introduce to can mean to insert or include).
'the second of horses' becomes 'o' (2nd letter of 'horses').
'try' placed around 'o' is 'TROY'.
(Other definitions for troy that I've seen before include "Ancient city associated with a wooden horse" , "City of Paris" , "System of weights introduced by French" , "Besieged city which fell to Wooden Horse" , "Ancient city of the Iliad" .)