Monkey's first in orbit seeing the light (4)
I believe the answer is:
lamp
'the light' is the definition.
(lamp is a kind of light)
'monkey's first in orbit' is the wordplay.
'monkey's first' becomes 'm' (1st letter of 'monkey').
'in' is an insertion indicator.
'orbit' becomes 'lap' (I've seen this in another clue).
'm' put within 'lap' is 'LAMP'.
'seeing' acts as a link.
(Other definitions for lamp that I've seen before include "bulb" , "Florence Nightingale was the Lady with the . . . ." , "Source of artificial light" , "punch" , "Table light" .)