RAF crew are almost then involved in land battle (6,7)
I believe the answer is:
trench warfare
'land battle' is the definition.
(battle using trenches)
'raf crew are almost then involved' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'raf'+'crew'+'are'='rafcreware'
'rafcreware' with its last letter taken away is 'rafcrewar'.
'rafcrewar'+'then'='rafcrewarthen'
'rafcrewarthen' with letters rearranged gives 'TRENCH WARFARE'.
'in' is the link.
(Other definitions for trench warfare that I've seen before include "Combat on Western Front in WW1" , "Combat from dug-in positions" , "combat in the field" .)