Space agency introducing cover for outermost custom zip (3,1,7)
I believe the answer is:
not a sausage
'zip' is the definition.
(I've seen this before)
'space agency introducing cover for outermost custom' is the wordplay.
'space agency' becomes 'nasa' (I've seen this before).
'introducing' indicates putting letters inside (some letters introduced or inserted into others).
'cover for' suggests removing the centre.
'custom' becomes 'usage' (synonyms).
'outermost' with its centre removed is 'ot'.
'nasa' placed around 'ot' is 'notasa'.
'notasa'+'usage'='NOT A SAUSAGE'
(Other definitions for not a sausage that I've seen before include "Alternative to banger, maybe" , "diddly-squat" , "zip" , "Absolutely nothing" , "Zilch (colloquial)" .)