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 justify this - if you can you should give a lot more credence to this answer).
'full of' is an insertion indicator.
'english' becomes 'E' (abbreviation as in OED).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'it' backwards is 'ti'.
'e'+'ti'='eti'
'sommes' enclosing '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" .)