Try to have English force atop castle (6)
I believe the answer is:
effort
'try' is the definition.
(I know that try can be written as effort)
'english force atop castle' is the wordplay.
'english' becomes 'e' (abbreviation).
'force atop' becomes 'f' (symbol used in Physics e.g. f=ma. I am not sure about the 'atop' bit.).
'castle' becomes 'fort' (fort is a type of castle).
'e'+'f'+'fort'='EFFORT'
'to have' is the link.
Can you help me to learn more?
(Other definitions for effort that I've seen before include "Endeavour, elbow grease" , "Try - exertion" , "For eft (anag.)" , "A bid or endeavour" , "Exertion, elbow grease" .)