Soldiers advanced tracks around delta in plain (8)
I believe the answer is:
ordinary
'plain' is the definition.
(I've seen this before)
'soldiers advanced tracks around delta in' is the wordplay.
'soldiers' becomes 'OR' (Other Ranks).
'advanced' becomes 'a' (abbreviation).
'tracks' becomes 'ry' (abbreviation for railway).
'around' is an insertion indicator.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'or'+'a'+'ry'='orary'
'd'+'in'='din'
'orary' enclosing 'din' is 'ORDINARY'.
(Other definitions for ordinary that I've seen before include "'Commonplace, banal (8)'" , "Unexceptional, quite usual" , "Workaday" , "pedestrian" , "Common-or-garden" .)