Occasionally battlefield's full of English climbing it (9)
I believe the answer is:
sometimes
'occasionally' is the definition.
(I've seen this before)
'battlefield's full of english climbing it' is the wordplay.
'battlefield's' becomes 'sommes' (I can't explain this - if you can you should believe this answer much more).
'full of' indicates putting letters inside.
'english' becomes 'E' (abbreviation).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'it' written backwards gives 'ti'.
'e'+'ti'='eti'
'sommes' going around 'eti' is 'SOMETIMES'.
Can you help me to learn more?
(Other definitions for sometimes that I've seen before include "Occasionally available" , "Seem moist (anag.)" , "On occasion" , "Not always" .)