Funny (not the first) thing astronomer looks into (6)
I believe the answer is:
ocular
'thing astronomer looks into' is the definition.
(ocular can mean eyepiece)
'funny not the first' is the wordplay.
'funny' becomes 'jocular' (I've seen this before).
'not the first' means to remove the first letter.
'jocular' with its first letter removed is 'OCULAR'.
(Other definitions for ocular that I've seen before include "Relating to the eye" , "Of the eye - visible" , "not to be seen - due to this trouble?" , "Glaucoma's such" , "Eyepiece" .)