English friend the writer would put first, in theory (7)
I believe the answer is:
ideally
'in theory' is the definition.
(thesaurus)
'english friend the writer would put first' is the wordplay.
'english' becomes 'e' (abbreviation).
'friend' becomes 'ally' ('ally' can be a synonym of 'friend').
'the writer' becomes 'i' (the person who is writing these clues).
'would' becomes 'd' (I've seen this in other clues).
'put first' means one lot of letters go next to another.
'i'+'d'='id'
'e'+'ally'='eally'
'eally' put after 'id' is 'IDEALLY'.
(Other definitions for ideally that I've seen before include "In the most perfect way" , "for preference" , "In the best possible scenario" , "Lay idle (anag)" , "in theory" .)