Sailor seen in minimal weak illumination (9)
I believe the answer is:
starlight
'weak illumination' is the definition.
'starlight' can be an answer for 'illumination' (I have seen 'faint illumination' mean 'starlight' so perhaps 'illumination' could also mean 'starlight'). I am unsure of the 'weak' bit.
'sailor seen in minimal' is the wordplay.
'sailor' becomes 'tar' (tar can mean a sailor).
'seen in' is an insertion indicator.
'minimal' becomes ''slight' (I've seen this in another clue).
'tar' inserted inside 'slight' is 'STARLIGHT'.
Can you help me to learn more?
(Other definitions for starlight that I've seen before include "faint glow" , "nocturnal illumination" , "faint glimmer" , "Dim heavenly illumination" , "Express musically this" .)