Submit to first getting one tube (4,2)
I believe the answer is:
give in
'submit' is the definition.
(I know that submit can be written as give in)
'first getting one tube' is the wordplay.
'first getting' becomes 'g' (ist letter of 'getting').
'one' becomes 'i' (Roman numeral).
'tube' becomes 'vein' (vein is a kind of tube).
'g'+'i'+'vein'='GIVE IN'
'to' is the link.
(Other definitions for give in that I've seen before include "Surrender, cease to resist" , "Cease arguing; submit" , "Admit defeat" , "Surrender or hand over" , "Surrender, yield" .)