splitting at underscore in python and storing the first value

Just use the vectorised str method split and use integer indexing on the list to get the first element:

In [228]:

df['first'] = df['construct_name'].str.split('_').str[0]
df
Out[228]:
  construct_name first
0      aaaa_t1_2  aaaa
1     cccc_t4_10  cccc
2      bbbb_g3_3  bbbb