New recruits I sent out to hold line - observe, in retreat (9)

'new recruits' is the definition.
'enlistees' can be an answer for 'recruits' ('enlistee' can be a synonym of 'recruit'). I'm unsure of the 'new' bit.

'i sent out to hold line observe in retreat' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'to hold' is an insertion indicator.
'line' becomes 'l' (used when specifying particular lines from a poem).
'observe' becomes 'see' (I've seen this before).
'in retreat' is a reversal indicator.
'see' written backwards gives 'ees'.
'i'+'sent'='isent'
'isent' is an anagram of 'enist'.
'enist' going around 'l' is 'enlist'.
'enlist'+'ees'='ENLISTEES'

Can you help me to learn more?