Ground team splits up to bombard enemy lines? (4,4)
I believe the answer is:
hate mail
'bombard enemy lines?' is the definition.
I know nothing about this answer so I can't judge whether this works.
'ground team splits up' is the wordplay.
'ground' indicates an anagram ('grind' the letters into a new form).
'splits' means one lot of letters goes inside another (inserted letters split open the word).
'up' becomes 'hail' (I can't justify this - if you can you should believe this answer much more).
'team' with letters rearranged gives 'atem'.
'atem' inserted inside 'hail' is 'HATE MAIL'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for hate mail that I've seen before include "communication with animosity - with 19a" , "Offensive correspondence" , "Poison-pen letters" , "Abusive letter that expresses loathing" , "Nasty letters" .)