Compilers had to pen gag -- it's very poor (8)
I believe the answer is:
wretched
'very poor' is the definition.
'wretched' can be an answer for 'poor' (synonyms). I'm unsure of the 'very' bit.
'compilers had to pen gag' is the wordplay.
'compilers' becomes 'we' (the crossword compiler and their colleagues).
'had' becomes 'd' (in contractions e.g. 'I'd', 'you'd' etc).
'to pen' means one lot of letters goes inside another.
'gag' becomes 'retch' (I've seen this before).
'we'+'d'='wed'
'wed' going around 'retch' is 'WRETCHED'.
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for wretched that I've seen before include "Pitiful" , "Abject" , "Poor" , "Sorry!" , "misery" .)