Gut reaction, ultimately admit love at the start (7)
I believe the answer is:
enteron
'gut' is the definition.
(I've seen this before)
'reaction ultimately admit love at the start' is the wordplay.
'ultimately' suggests the final letters.
'admit' becomes 'enter' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'love' becomes 'o' (love means zero in tennis).
'at the start' means one lot of letters go next to another.
The final letter of 'reaction' is 'n'.
'enter'+'o'='entero'
'n' after 'entero' is 'ENTERON'.
Can you help me to learn more?
(Other definitions for enteron that I've seen before include "element of guts" , "Begin; engage in" , "gut" , "Open" .)