Here are some possible translations depending on the missing word:
If it's "comme un ami":
* "I miss you, but only as a friend."
If it's "comme la pluie en été":
* "I miss you, but only like rain in the summer." (This would be a very poetic and metaphorical way to express missing someone.)
If it's "comme un livre que j'ai déjà lu":
* "I miss you, but only like a book I've already read." (This implies that while there are fond memories, the longing isn't as intense as it might be for a new experience.)
In general:
* "I miss you, but only..."
To get a more accurate translation, you need the full phrase with the missing word.