First male worker was unyielding (7)
'unyielding' is the definition.
(I've seen this before)
'first male worker' is the wordplay.
'first male' becomes 'adam'.
'worker' becomes 'ant' (worker is a type of ant).
'was' is the link.
(Other definitions for adamant that I've seen before include "Fully determined", "Obdurate and unyielding", "Inflexible: fabulously hard rock", "Refusing to budge", "Firm and unyielding".)